doc: genrest: Use a separate index page for all symbols

Add an option --separate-all-index that makes genrest.py generate a
separate index-all.rst index page that lists all symbols, instead of
listing all symbols in index.rst. index-all.rst is linked from
index.rst.

This was originally motivated by an external project where index.rst
becomes the top-level page, which runs into a Sphinx bottleneck with
sphinx_rtd_theme. See https://github.com/sphinx-doc/sphinx/issues/6909

.

This turned out pretty nice after some feedback from Ruth Fuchss, so use
it for Zephyr too.

Also unclutter the generated documentation a bit by removing some
headings. This makes the navigation menu on the left nicer too.

Piggyback making genrest.py executable, which is handy when running it
manually.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>