gendoc: add support for loading extensions from config settings
We manage our installation and ship some extensions, enabled by default for our
users, that are in hgext3rd or other directories not scanned by this tool by
default. We want to generate docs during the build process, and having those
docs include the extensions that users don't have to manually enable is
This is *not* desirable for the normal build process, however, and should never
be enabled by default.
Differential Revision: https://phab.mercurial-scm.org/D9989