maintainer-scripts: build the libgdiagnostics docs for the website [PR117883]

maintainer-scripts/ChangeLog:
	PR web/117883
	* update_web_docs_git: Introduce SPHINX_VENV to make
	it easier to test the script.  Add the libgdiagnostics docs
	and testsuite to the files to be preserved.  Use sphinx to build
	the libgdiagnostics docs as HTML.  Copy them into $DOCSDIR.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>