[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>

docs/admin/engines/sql-engines.rst

@@ -136,12 +136,10 @@ find an example configuration below:
 MySQL
 -----
 
-.. _mysql-connector-python: https://pypi.org/project/mysql-connector-python
-
 .. sidebar:: info
 
    - :origin:`mysql_server.py <searx/engines/mysql_server.py>`
-   - ``pip install`` mysql-connector-python_
+   - ``pip install`` :pypi:`mysql-connector-python <mysql-connector-python>`
 
 MySQL is said to be the most popular open source database. Before enabling MySQL
 engine, you must install the package ``mysql-connector-python``.

docs/admin/installation-switch2ng.rst

@@ -7,7 +7,7 @@ Switch from searx to SearXNG
 .. sidebar:: info
 
    - :pull:`456`
-   - `A comment about rolling release <https://github.com/searxng/searxng/pull/446#issuecomment-954730358>`_
+   - :pull:`A comment about rolling release <446#issuecomment-954730358>`
 
 .. contents:: Contents
    :depth: 2