ci: add job to build docs and trigger handbook pipeline for deployment

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7065

[ci:skip-build]: already built successfully in CI

.ci/build-docs.sh

@@ -0,0 +1,22 @@
+#!/bin/sh -e
+# Copyright 2025 Pablo Correa Gomez
+# SPDX-License-Identifier: GPL-3.0-or-later
+# Description: create documentation with sphinx
+# Options: with-dot-git
+# Artifacts: build/site
+# https://postmarketos.org/pmb-ci
+
+
+# Install dependencies
+if [ "$(id -u)" = 0 ]; then
+	set -x
+	apk -q add git nodejs npm
+	exec su "${TESTUSER:-build}" -c "sh -e $0"
+fi
+
+npm install antora
+npx antora -v
+
+# FIXME: https://gitlab.com/antora/antora/-/issues/1183
+git clone --depth 1 https://gitlab.postmarketos.org/postmarketOS/handbook
+npx antora --clean antora-test.yml