+ Use the hotdoc autotools helpers

This requires using -Wno-portability in the call to AM_INIT_AUTOMAKE,
as discussed with Simon it's a reasonable requirement for using the
hotdoc.mk fragment Makefile.

The produced html is now cleanly installable and uninstallable.

+ Make documentation build parallel for croesor and rhosydd

The issue preventing this is fixed upstream, although in a
different manner than the linked github issue proposed.

+ Remove distribution of the generated documentation

Distributing build products in the source tarball is a bad practice,
and there's no compelling reason for doing this in Apertis libraries.

+ Use a sitemap

This means subpages do not need to be linked and updated in markdown
files.

+ Enable ‘smart index’

+ Enable devhelp books generation

+ Update the debian files

+ remove build-depends on hotdoc-dbus-extension, not sure why it
  was added in the first place.

Using this new feature of hotdoc means we no longer need to maintain
potentially huge lists of symbols in the markdown files.

As desired by Philip, we now pass the C sources to hotdoc in the
Makefile, which means the source list does not need updating.

However, combined with smart-index, this means every public symbol
is now exposed in the documentation.

Subsequent patches should filter out potentially undesirable files
or symbols, this requires more in-depth knowledge of the project
than I currently possess.

Signed-off-by: Mathieu Duponchelle <mathieu.duponchelle@opencreed.com>

Differential Revision: https://phabricator.apertis.org/D3784