Add docs on module level or global pytest fixture configuration

[JacobHayes]

I wanted to override assert_all_called=True for my entire test suite, but couldn't quite see how from the docs. This PR adds info on how to override the respx_mock fixture at both the module level and globally.

[lundberg]

How to globally enable respx is a great suggestion to the docs.

Normally I use an auto-used session scoped fixture for this, e.g. @pytest.fixture(scope="session", autouse=True). An autouse fixture may be simpler a more common test pattern (?).

Maybe the docs on custom fixtures would be a better place to include an example at .. possibly with a link from the guide.

[JacobHayes]

Hmm yeah an autouse fixture is probably better for most uses.

In my case, I didn't want to affect httpx in tests that weren't otherwise mocking, so an autouse fixture (which would add mocking) wasn't ideal.