[docs] Update docuementation about webkitgtk and wpewebkit runners
This is a follow-up to commit c9da6f7 to fix some missing backticks
in the wpewebkit_minibrowser documentation.
Seize this to improve a bit the text of the documentation as well as
the rendered format.
Also update the documentation on how to build the docs locally that
was outdated since a7c2f53