Add GHA workflow for checking valid HTML5 docs

To know more about new HTML5 check in R-devel, see:
https://blog.r-project.org/2022/04/08/enhancements-to-html-documentation/

Workflow from:
https://github.com/r-lib/actions/blob/v2-branch/examples/html-5-check.yaml

Author: IndrajeetPatil

.github/workflows/html-5-check.yaml

@@ -0,0 +1,44 @@
+# Workflow derived from https://github.com/r-lib/actions/tree/v2/examples
+# Need help debugging build failures? Start at https://github.com/r-lib/actions#where-to-find-help
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+name: HTML5 check
+
+jobs:
+  HTML5-check:
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
+      R_KEEP_PKG_SOURCE: yes
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: r-lib/actions/setup-r@v2
+        with:
+          r-version: 'devel'
+          http-user-agent: 'release'
+          use-public-rspm: true
+
+      - uses: r-lib/actions/setup-r-dependencies@v2
+        with:
+          extra-packages: any::rcmdcheck
+          dependencies: 'character()'
+
+      - name: Install pdflatex
+        run: sudo apt-get install texlive-latex-base texlive-fonts-recommended texlive-fonts-extra texlive-latex-extra
+
+      - name: Install tidy
+        run: sudo apt install tidy
+
+      - uses: r-lib/actions/check-r-package@v2
+        with:
+          args: 'c("--as-cran", "--no-codoc", "--no-examples", "--no-tests", "--no-vignettes", "--no-build-vignettes", "--ignore-vignettes", "--no-install")'
+          build_args: 'c("--no-build-vignettes")'
+          error-on: '"note"'
+        env:
+          _R_CHECK_CRAN_INCOMING_REMOTE_: false
+          _R_CHECK_CRAN_INCOMING_: false