docs: Copy in API docs and add link

docs: Copy in API docs and add link

Make a copy of `apidoc/html` to `docs/reference` and then tell Jekyll to
include it verbatim. This will include the gtk-doc API docs on the
static site. A link is added to the main index.

A script is added to do the copy (a symlink won't do) and is setup to
run before Jekyll in the GitHub workflow. Ideally this would be a local
Jekyll plugin to make the process automatic, but the github-pages gem
doesn't allow that.