man: convert to reStructuredText

DocBook makes it hard to write and maintain docs. Hopefully
reStructuredText can make this less painful.

The man pages were converted from DocBook to reStructuredText via
Pandoc:

    pandoc -s -f docbook -t rst -o man/drm.7.rst man/drm.xml

And then manual editing to fixup e.g. references to other man pages. To
compare the result with the DocBook version, this command was used:

    rst2man man/drm-kms.7.rst | man -l -

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>