docs: configure default_role=literal

[blueyed]

This configures the default role for interpreted text (single
backticks), avoiding the need to check for / enforce double backticks.

Fixes also one instance in the existing changelog:

- Detect `pytest_` prefixed hooks using the internal plugin manager since
  ``pluggy`` is deprecating the ``implprefix`` argument to ``PluginManager``.
  (`#3487 <https://github.com/pytest-dev/pytest/issues/3487>`_)

[nicoddemus]

Ohh cool, good find! This should go into master though so we can benefit from it right away.