ci: Build docs in PRs and in private repos (#6400)

.github/workflows/publish-docs.yml

@@ -4,6 +4,18 @@ name: Build and publish documentation
 
 on:
   push:
+    branches:
+      - "develop"
+      - "release*"
+    paths:
+      - ".github/workflows/publish-docs.yml"
+      - "*.md"
+      - "**/*.md"
+      - "docs/**"
+      - "include/**"
+      - "src/libxrpl/**"
+      - "src/xrpld/**"
+  pull_request:
     paths:
       - ".github/workflows/publish-docs.yml"
       - "*.md"
@@ -23,7 +35,9 @@ defaults:
 
 env:
   BUILD_DIR: build
-  NPROC_SUBTRACT: 2
+  # ubuntu-latest has only 2 CPUs for private repositories
+  # https://docs.github.com/en/actions/reference/runners/github-hosted-runners#standard-github-hosted-runners-for--private-repositories
+  NPROC_SUBTRACT: ${{ github.event.repository.private && '1' || '2' }}
 
 jobs:
   publish:
@@ -65,7 +79,7 @@ jobs:
           cmake --build . --target docs --parallel ${BUILD_NPROC}
 
       - name: Publish documentation
-        if: ${{ github.ref_type == 'branch' && github.ref_name == github.event.repository.default_branch }}
+        if: ${{ github.event_name == 'push' }}
         uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e # v4.0.0
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}