Add script for building GitHub pages to branch

.gitignore

@@ -1,2 +1,3 @@
 /.user.bazelrc
 /bazel-*
+/gh-pages

docs/push.sh

@@ -0,0 +1,49 @@
+#!/bin/sh
+# Build the site and push it to GitHub pages.
+set -e
+
+root=$(git rev-parse --show-toplevel)
+cd "$root"
+
+if ! command -v frum >/dev/null ; then
+  echo >&2 "Error: frum is not installed"
+  exit 1
+fi
+
+if ! git diff-index --quiet --cached HEAD -- docs ; then
+  echo >&2 "Error: uncommitted changes"
+  exit 1
+fi
+if ! git diff-files --quiet docs ; then
+  echo >&2 "Error: uncommitted changes"
+  exit 1
+fi
+branch=$(git symbolic-ref HEAD)
+if test "$branch" != refs/heads/main ; then
+  echo >&2 "Error: branch is not main"
+  exit 1
+fi
+commit="$(git rev-parse HEAD)"
+
+echo >&2 "Checking out gh-pages..."
+if test -d gh-pages ; then
+  branch=$(git -C gh-pages symbolic-ref HEAD)
+  if test "$branch" != refs/heads/gh-pages ; then
+    echo >&2 "Error: gh-pages dir does not have gh-pages branch"
+    exit 1
+  fi
+else
+  git worktree add gh-pages
+fi
+
+echo >&2 "Building site..."
+(
+  cd docs
+  eval "$(sh -c 'frum init')"
+  bundle exec jekyll build --destination ../gh-pages
+)
+
+echo >&2 "Committing..."
+cd gh-pages
+git add .
+git commit -m "Generated from commit ${commit}"