Convert man page highlighting to standard RST

Instead of using the sphinx-specific highlight extension, which is not
parsed by Pandoc, let's switch to the standard RST directive, which
will be picked up and will result in slightly nicer man pages when
converted to HTML (in man output it remains the same).

Note that I've converted only some of the examples (the one that
actually had shell scripts); the ones that show command lines
(e.g. starting with '#') I didn't convert, as they don't look nice
(and Pandoc/Kate highlighting doesn't have the equivalent of our
shell-example lexer we use in sphinx).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>