rt-static-docs: Rename --rt to --source

In preparation for processing docs in RT extensions.
Save what our POD search found and use that info when making local links

If we find a link to an RT resource but didn't find the referenced name
in the POD search, then assume we're documenting an extension and link
one level up into rt/.  This allows links in extensions which point to
core documentation.
Fix two POD linking errors uncovered by the new stricter local linking
Document RT::CustomFieldValues::Groups so links resolve correctly to it
Link the latest core RT docs from non-core docs
rt-static-docs: Add an --extension flag which doesn't do much (yet)

Currently it just suppresses a warning about not finding ./configure.
If we didn't find a local link in the current source, it needs to link even higher up the tree

This accounts for the version component of the docs tree.
POD links to RT::Extension:: and RT::Authen:: are not local
Any RT::Extension:: or RT::Authen:: docs we found during this batch _are_ local

This also resolves
    L<Tutorial/Getting started>
to
    Tutorial.html#Getting-started
if a Tutorial.pod was found (such as in docs/).
Link extension config files when in F<>

This lets F<RTIR_Config.pm> work just like F<RT_Config.pm>.
Classify RTIR_Config and other extension configs as install docs

Alongside core RT_Config
Merge branch '4.0/extension-docs' into 4.0-trunk