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

@@ -0,0 +1,34 @@
+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