Preview changes

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# Without --id (unique UUID each time)
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[docs]: Published docs to https://fern-preview-c973a36e-337b-44f5-ab83-aab.docs.buildwithfern.com/learn
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# With --id my-feature (stable URL)
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[docs]: Published docs to https://fern-preview-my-feature.docs.buildwithfern.com/learn

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name: Preview Docs
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on:
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  pull_request:
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    types: [opened, synchronize, ready_for_review]
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    branches:
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      - main
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jobs:
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  run:
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    runs-on: ubuntu-latest
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    permissions:
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      pull-requests: write
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      contents: read
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    steps:
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      - name: Checkout repository
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        uses: actions/checkout@v4
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        with:
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          fetch-depth: 0
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      - name: Setup Fern CLI
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        uses: fern-api/setup-fern-cli@v1
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      - name: Generate preview URL
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        id: generate-docs
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        env:
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          FERN_TOKEN: ${{ secrets.FERN_TOKEN }}
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          HEAD_REF: ${{ github.head_ref }}
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        run: |
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          OUTPUT=$(fern generate --docs --preview --id "$HEAD_REF" 2>&1) || true
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          echo "$OUTPUT"
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          URL=$(echo "$OUTPUT" | grep -oP 'Published docs to \K.*(?= \()')
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          echo "preview_url=$URL" >> $GITHUB_OUTPUT
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          echo "Preview URL: $URL"
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      - name: Get page links for changed MDX files
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        id: page-links
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        env:
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          FERN_TOKEN: ${{ secrets.FERN_TOKEN }}
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        run: |
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          PREVIEW_URL="${{ steps.generate-docs.outputs.preview_url }}"
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          CHANGED_FILES=$(git diff --name-only origin/main...HEAD -- '*.mdx' 2>/dev/null || echo "")
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          if [ -z "$CHANGED_FILES" ] || [ -z "$PREVIEW_URL" ]; then
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            echo "page_links=" >> $GITHUB_OUTPUT; exit 0
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          fi
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          BASE_URL=$(echo "$PREVIEW_URL" | grep -oP 'https?://[^/]+')
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          FILES_PARAM=$(echo "$CHANGED_FILES" | tr '\n' ',' | sed 's/,$//')
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          RESPONSE=$(curl -sf -H "FERN_TOKEN: $FERN_TOKEN" "${PREVIEW_URL}/api/fern-docs/get-slug-for-file?files=${FILES_PARAM}" 2>/dev/null) || {
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            echo "page_links=" >> $GITHUB_OUTPUT; exit 0
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          }
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          PAGE_LINKS=$(echo "$RESPONSE" | jq -r --arg url "$BASE_URL" \
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            '.mappings[] | select(.slug != null) | "- [\(.slug)](\($url)/\(.slug))"')
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          if [ -n "$PAGE_LINKS" ]; then
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            { echo "page_links<<EOF"; echo "$PAGE_LINKS"; echo "EOF"; } >> $GITHUB_OUTPUT
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          else
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            echo "page_links=" >> $GITHUB_OUTPUT
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          fi
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      - name: Create comment content
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        run: |
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          echo ":herb: **Preview your docs:** <${{ steps.generate-docs.outputs.preview_url }}>" > comment.md
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          if [ -n "${{ steps.page-links.outputs.page_links }}" ]; then
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            echo "" >> comment.md
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            echo "Here are the markdown pages you've updated:" >> comment.md
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            echo "${{ steps.page-links.outputs.page_links }}" >> comment.md
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          fi
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      - name: Post PR comment
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        uses: thollander/actions-comment-pull-request@v2.4.3
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        with:
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          filePath: comment.md
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          comment_tag: preview-docs
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          mode: upsert