Allow switching doc examples between auto and strict mode

[seifertm]

Users of pytest-asyncio either use auto mode or strict mode. They cannot mix modes within the same project. Some code examples in the documentation are presented for each mode, but the user is only interested for the examples in one of the modes.

I propose to add sphinx-tabs to the doc dependencies and use group tabs to show only code examples for the selected pytest-asyncio mode.

Code examples that are only available for one mode should be provided for both, auto mode and strict mode as part of this issue.