Fix cross-referencing in gplugin docs

gi-docgen already puts types on most documented things. Thus most of this is replacing the linking with normal text.

But otherwise things are linked into GLib, internally, etc.


* Improve documentation of manager
For parameters, dropped the types because gi-docgen shows them automatically. It also shows transfer full, etc., so don't write extra notes about referencing and freeing.
The types are not correctly shown on signals, which _are_ kept in docs.
Also, break up some paragraphs so that the details are not shown in the summaries on the overview page for the Manager.
* Clean up loader docs
Most of the links are not needed, so this just makes them normal text.
* Fix doc cross-references in plugin info
Like before, most of this is actually _removing_ extra cross-references, but there are still one or two left.
* Clean up cross references in plugin interface
* Fix cross-referencing in top-level functions

Testing Done:
Compiled and looked at docs in the browser.

Reviewed at https://reviews.imfreedom.org/r/999/