CI: Use GitHub Actions to build/validate/publish (w3c#15)

This change adds CI automation for building, validating and publishing
the document using the https://github.com/w3c/spec-prod tooling.

Author: sideshowbarker

.github/workflows/build-validate-publish.yml

@@ -0,0 +1,18 @@
+name: CI
+on:
+  pull_request: {}
+  push:
+    branches: [main]
+
+jobs:
+  main:
+    name: Build, Validate, and Publish
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: w3c/spec-prod@v2
+        with:
+          GH_PAGES_BRANCH: gh-pages
+          BUILD_FAIL_ON: nothing
+          VALIDATE_LINKS: false
+          VALIDATE_MARKUP: true

.gitignore

@@ -0,0 +1 @@
+index.html