doc: Mesonize documentation generation

Introduce a new config flag -Ddocs-build=true which controls if the
build system should generate documentation or not. If disabled but
-Ddocs=man is used then the pre compiled documentation will be
installed.

Furthmore, we are hooking up the sphinx tooling so that we generate
the HTML documentation ouf of the rst files.

Signed-off-by: Daniel Wagner <[email protected]>

Author: dwsuse