Add github action to generate and push docs to docs branch

Instead of requiring that users regenerate auto-generated documentation
each time it changes, this adds a github action to force push any
changes on the main branch to the docs branch.  Github pages allows
docs to be on any branch, in the docs folder.

.github/workflows/gen_docs.yaml

@@ -0,0 +1,18 @@
+name: Generate Documenation Pages
+on:
+  push:
+    branches:
+      - main
+jobs:
+  generate:
+    # if: ${{ github.repository_owner == 'bazelbuild' }}
+    name: Generate Docs
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install bazelisk
+        run: .github/workflows/scripts/install_bazelisk.sh
+        env:
+          USE_BAZEL_VERSION: 4.0.0
+      - name: Generate and Push
+        run: .github/workflows/scripts/publish_docs.sh

.github/workflows/install_bazelisk.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -euo pipefail
+
+curl -LO "https://github.com/bazelbuild/bazelisk/releases/download/v1.1.0/bazelisk-linux-amd64"
+mkdir -p "${GITHUB_WORKSPACE}/bin/"
+mv bazelisk-linux-amd64 "${GITHUB_WORKSPACE}/bin/bazel"
+chmod +x "${GITHUB_WORKSPACE}/bin/bazel"

.github/workflows/publish_docs.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+set -exuo pipefail
+
+# run within the docs workspace
+pushd docs &> /dev/null
+
+# We use a bazelisk-installed bazel
+"${GITHUB_WORKSPACE}/bin/bazel" clean
+"${GITHUB_WORKSPACE}/bin/bazel" build //...
+
+# Pull out the relevant docs
+cp bazel-bin/*.md .
+chmod 0644 *.md
+
+# And generate a new commit, that we push to the "docs" branch.
+git config user.name github-actions
+git config user.email github-actions@github.com
+git add *.md
+git commit -m "Regenerate documentation"
+git remote -v
+git push -f origin HEAD:docs
+
+popd &> /dev/null
+