[docs] fix some warnings from Sphinx-doc update 4.3.2 to 4.4.0

With Sphinx-doc update 4.4.0 we get some warnings about links that can be
replaced by already defined 'sphinx.ext.extlinks':

    admin/engines/sql-engines.rst:144: WARNING: hardcoded link 'https://pypi.org/project/mysql-connector-python' could be replaced by an extlink (try using ':pypi:`mysql-connector-python`' instead)
    docs/admin/installation-switch2ng.rst:10: WARNING: hardcoded link 'https://github.com/searxng/searxng/pull/446#issuecomment-954730358

' could be replaced by an extlink (try using ':pull:`446#issuecomment-954730358`' instead)

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