docs: Port from gtk-doc to gi-docgen

gtk-doc has been slowly dying for the past few years. With gi-docgen we
have a clear successor in sight to replace the voodoo magic one needs to
get the whole documentation setup.

See the [gi-docgen tutorial] for more info on how the system works.

Since we're now only the C compiler (and GIR) parse the C code, that
means we can also get rid of all the special escapes for the # character
in PKCS#11.

[gi-docgen tutorial]: https://gnome.pages.gitlab.gnome.org/gi-docgen/tutorial.html