Help screen index missing

Since 3.2.0, new site installs are missing the help screen index if you go to `example.com/administrator/index.php?option=com_admin&view=help`.  This is because of https://github.com/joomla/joomla-cms/commit/ae3e37dda756433e4c2ed6286efc7a5ba233baa2 in which the static files which generated the index being removed.  Since these files weren't removed via the update script, sites updating from 3.1 or earlier still have this index.

To rectify this without adding back several dummy files, I've written a script which can fetch the index directly from our docs wiki and added support for this new index to com_admin.  Essentially, this script fetches all pages from `docs.joomla.org/Category:Help_screen_<version>` and filters out all pages for which `COM_ADMIN_HELP` language strings do not exist (as this index only displays select views, not the entire category listing).

In order to fetch the data, a new method, `getCategoryMembers()` is added to `JMediawikiCategories` enabling support for this API endpoint.