(bug 45845) Allow for docs at non-subpages

It has been claimed that some wiki might want to place module
documentation at a location other than as a subpage of the module, for
example under "Project:Module documentation/$1". It's possible to
support this, so we may as well.

This also involves renaming the "scribunto-doc-subpage-*" messages to
"scribunto-doc-page-*", since the interpretation of
scribunto-doc-subpage-name would be drastically changed.

Note that any wiki that has customized scribunto-doc-subpage-name will
need to re-customize scribunto-doc-page-name, the old value will not be
transferred.

Bug: 45845
Change-Id: Ic453561691e04b5250d219cc7d871c17e60b9912
Merge "(bug 45845) Allow for docs at non-subpages"