Reorder the reference guides in the docs
Reorder the reference guides in the docs

This changes the order to:

- API Reference
- Fixtures reference
- Configuration
- Exit codes
- Plugin List

which is approximately sorted from general to specific and often used to less used.
Plugin List ist at the end because it points to further external resources.

pytest-dev/pytest,RonnyPfannschmidt/pytest,markshao/pytest