docs: Port the man pages from DocBook to reStructuredText

So they are consistent with the way we’re building man pages in other projects, and because some people are allergic to XML.

This changes the build-time dependencies from xsltproc to rst2man, and also takes the opportunity to change the -Dman Meson option from a boolean to a feature (so you should use -Dman={enabled,disabled} now, rather than -Dman={true,false}).

Signed-off-by: Philip Withnall pwithnall@gnome.org

Helps: #3037 (closed)