documentation/doxygen: omit filename in anchors to the page itself.

documentation/doxygen: omit filename in anchors to the page itself.

I'm already doing that for all links coming from the template
(crosslinks between member lists and detailed info, section heading
links etc.) but so far it wasn't done for links and references contained
in Doxygen comments themselves.

That not only makes the generated files smaller, especially when there's
a lot of cross-references with large namespace docs full of functions
and typedefs, but also makes the output consistent between 1.8.16 and
1.12, which is why I'm doing this in the first place.