doc: use customized pocoo theme (pallets-sphinx-themes)

The old searx theme was a copy of the flask theme.  In the meantime this theme
is available from the python module pallets-sphinx-themes.

This patch makes pallets-sphinx-themes as a (dev) requirement and drops most of
the old and obsolete searx theme settings/files.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>