Upload documentation PRs to Netlify (#12947)

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>
Co-authored-by: Erik Johnston <erik@matrix.org>
Co-authored-by: David Robertson <davidr@element.io>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

.github/workflows/docs-pr-netlify.yaml

+name: Deploy documentation PR preview
+
+on:
+  workflow_run:
+    workflows: [ "Prepare documentation PR preview" ]
+    types:
+      - completed
+
+jobs:
+  netlify:
+    if: github.event.workflow_run.conclusion == 'success' && github.event.workflow_run.event == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+      # There's a 'download artifact' action, but it hasn't been updated for the workflow_run action
+      # (https://github.com/actions/download-artifact/issues/60) so instead we get this mess:
+      - name: 📥 Download artifact
+        uses: dawidd6/action-download-artifact@af92a8455a59214b7b932932f2662fdefbd78126 # v2.15.0
+        with:
+          workflow: docs-pr.yaml
+          run_id: ${{ github.event.workflow_run.id }}
+          name: book
+          path: book
+
+      - name: 📤 Deploy to Netlify
+        uses: matrix-org/netlify-pr-preview@v1
+        with:
+          path: book
+          owner: ${{ github.event.workflow_run.head_repository.owner.login }}
+          branch: ${{ github.event.workflow_run.head_branch }}
+          revision: ${{ github.event.workflow_run.head_sha }}
+          token: ${{ secrets.NETLIFY_AUTH_TOKEN }}
+          site_id: ${{ secrets.NETLIFY_SITE_ID }}
+          desc: Documentation preview
+          deployment_env: PR Documentation Preview

.github/workflows/docs-pr.yaml

+name: Prepare documentation PR preview
+
+on:
+  pull_request:
+    paths:
+      - docs
+
+jobs:
+  pages:
+    name: GitHub Pages
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup mdbook
+        uses: peaceiris/actions-mdbook@4b5ef36b314c2599664ca107bb8c02412548d79d # v1.1.14
+        with:
+          mdbook-version: '0.4.17'
+
+      - name: Build the documentation
+        # mdbook will only create an index.html if we're including docs/README.md in SUMMARY.md.
+        # However, we're using docs/README.md for other purposes and need to pick a new page
+        # as the default. Let's opt for the welcome page instead.
+        run: |
+          mdbook build
+          cp book/welcome_and_overview.html book/index.html
+
+      - name: Upload Artifact
+        uses: actions/upload-artifact@v2
+        with:
+          name: book
+          path: book
+          # We'll only use this in a workflow_run, then we're done with it
+          retention-days: 1

changelog.d/12947.doc

+Upload documentation PRs to Netlify.