docs: Use wrapper to gi-docgen
As we need to "fix" files generated by gi-docgen (namely, the devhelp2 and index.json files so they include our ontology keywords), do this as a wrapper script, instead of late as a install script.
This allows us to use install:
on the generated files like other
gi-docgen users, instead of relying on our own install script.
Related: #467 (closed)