Merge topic branch t/doxygen into pu

Merge topic branch t/doxygen into pu

This revamps the doxygen part of the documentation web page, adding a couple
of entry points for users to get started with the code base. We provide links
to header files which descripe the API of central subsystems and links to
the main file of each paraslash executable.

The \file comment of the source files reachable through these links have been
extended to contain a detailed description of the API or executable. Unlike
the introductory text of the manual pages, these comments focus on the code,
particularly the interaction between the various subsystems.

This also reactivates the source browser, since it is actually quite useful
to quickly check for some detail.

<!--

- Major revamp of the source code documention and the documentation web page.

-->

* refs/heads/t/doxygen: (38 commits)
  crypt
  gui
  doc-page
  f
  filter api
  recv
  afs
  afs
  afs
  fec
  doc-site
  send.h
  senders
  send/source-browser
  write
  afh
  afh.h
  f
  afh
  Move detailed desc to top
  ...

# Conflicts:
#	crypt.h