build: Use gnome-autogen.sh
Help: Add Mallard page for Alias plugin
Add AptURL doc page
Update help Makefile.am to actually do things
Move images in Mallard docs to media/ subdirectory
Fix help media links and install paths
Hook up docs for AptURL and Alias.

Point the ‘About’ button at the docs we install, rather than
the (spam, parked) do.davebsd.com wiki, or the non-existent wiki
on do.cooperteam.net
Merge start of Mallard documentation

Rather than relying on the wiki, that died under the morass of spam,
we can ship plugin documentation locally.