Enable automated update of gh-pagaes

Add 'scripts/github/update-gh-pages.sh' for updating gh-pages branch.
This new script intended for github actions integration and builds the
html documentation from the currently checked-out commit and updates the
corresponding version subfolder in the gh-pages branch. The version is
taken from GITHUB_REF environment variable and defaults to 'master' if
the env var is empty. For testing purposes or manual updates the
subfolder can also be given directly as a command line argument.

Add 'scripts/test-infra/build-gh-pages.sh' for verifying that site
(gh-pages) builds successfully. This is intended for integration with
kubernetes test-infra.

Finally, add Github workflow for updating gh-pages. This job is run on
master and release branches plus all tags starting with letter 'v'.

(cherry picked from commit c5beeffb6a)

.github/workflows/gh-pages.yml

@@ -0,0 +1,28 @@
+name: gh-pages
+on:
+  push:
+    branches:
+      - master
+      - release-*
+    tags:
+      - v*
+jobs:
+  build:
+    name: Update gh-pages
+    runs-on: ubuntu-latest
+    steps:
+
+    - name: Check out repo
+      uses: actions/checkout@v2
+
+    - name: Fetch gh-pages branch
+      run: git fetch --no-tags --prune --depth=1 origin refs/heads/gh-pages:refs/heads/gh-pages
+
+    - name: Build site
+      run: |
+        git config user.name "Github Actions"
+        git config user.email "no-reply@github.com"
+        ./scripts/github/update-gh-pages.sh
+
+    - name: Deploy site
+      run: git push -f https://${GITHUB_ACTOR}:${{ secrets.GITHUB_TOKEN }}@github.com/${{ github.repository }} gh-pages

scripts/github/update-gh-pages.sh

@@ -0,0 +1,125 @@
+#!/bin/bash -e
+set -o pipefail
+
+this=`basename $0`
+
+usage () {
+cat << EOF
+Usage: $this [-h] [-a] [SITE_SUBDIR]
+
+Options:
+  -h         show this help and exit
+  -a         amend (with --reset-author) instead of creating a new commit
+  -p REMOTE  do git push to remote repo
+EOF
+}
+
+#
+# Argument parsing
+#
+while getopts "hap:" opt; do
+    case $opt in
+        h)  usage
+            exit 0
+            ;;
+        a)  amend="--amend --reset-author"
+            ;;
+        p)  push_remote="$OPTARG"
+            ;;
+        *)  usage
+            exit 1
+            ;;
+    esac
+done
+shift "$((OPTIND - 1))"
+
+site_subdir="$1"
+
+# Check that no extra args were provided
+if [ $# -gt 1 ]; then
+    echo "ERROR: extra positional arguments: $@"
+    usage
+    exit 1
+fi
+
+#
+# Build the documentation
+#
+build_dir="docs/_site"
+echo "Creating new Git worktree at $build_dir"
+git worktree add "$build_dir" gh-pages
+
+# Drop worktree on exit
+trap "echo 'Removing Git worktree $build_dir'; git worktree remove '$build_dir'" EXIT
+
+# Parse subdir name from GITHUB_REF
+if [ -z "$site_subdir" ]; then
+    case "$GITHUB_REF" in
+        refs/tags/*)
+            _base_ref=${GITHUB_REF#refs/tags/}
+            ;;
+        refs/heads/*)
+            _base_ref=${GITHUB_REF#refs/heads/}
+            ;;
+        *) _base_ref=
+    esac
+    echo "Parsed baseref: '$_base_ref'"
+
+    case "$GITHUB_REF" in
+        refs/tags/v*)
+            _version=${GITHUB_REF#refs/tags/v}
+            ;;
+        refs/heads/release-*)
+            _version=${GITHUB_REF#refs/heads/release-}
+            ;;
+        *) _version=
+    esac
+    echo "Detected version: '$_version'"
+
+    _version=`echo -n $_version | sed -nE s'!^([0-9]+\.[0-9]+).*$!\1!p'`
+
+    # User version as the subdir
+    site_subdir=${_version:+v$_version}
+    # Fallback to base-ref i.e. name of the branch or tag
+    site_subdir=${site_subdir:-$_base_ref}
+fi
+
+# Default to 'master' if no subdir was given and we couldn't parse
+# it
+export SITE_SUBDIR=${site_subdir:-master}
+echo "Updating site subdir: '$SITE_SUBDIR'"
+
+make site-build
+
+#
+# Update gh-pages branch
+#
+if [ -n "$_GIT_TAG" ]; then
+    commit_hash=${GIT_TAG:10}
+else
+    commit_hash=`git describe --dirty --always`
+fi
+
+# Switch to work in the gh-pages worktree
+cd "$build_dir"
+
+if [ -z "`git status --short`" ]; then
+    echo "No new content, gh-pages branch already up-to-date"
+    exit 0
+fi
+
+# Create a new commit
+commit_msg=`echo -e "Update documentation for $SITE_SUBDIR\n\nAuto-generated from $commit_hash by '$this'"`
+
+echo "Committing changes..."
+git add .
+git commit $amend -m "$commit_msg"
+
+cd -
+
+echo "gh-pages branch successfully updated"
+
+if [ -n "$push_remote" ]; then
+    echo "Pushing gh-pages to $push_remote"
+    git push ${amend+-f} "$push_remote" gh-pages
+fi