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 desirable. 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