Introduce a dedicated section about conftest.py

[bilderbuchi]

Also reorganize conftest-related content a bit.
Closes #1505, closes #1770.

Btw, @nicoddemus, your PR template and CONTRIBUTING file are not consistent about if doc changes trigger/warrant an inclusion into the AUTHORS file.

[nicoddemus]

Thanks @bilderbuchi!

your PR template and CONTRIBUTING file are not consistent about if doc changes trigger/warrant an inclusion into the AUTHORS file.

What do you mean? Can you point out where they conflict with each other?

[nicoddemus]

Excellent addition, thanks!

[bilderbuchi]

Should have clarified immediately, sorry. One says only/plainly

Add yourself to AUTHORS file if not there yet, in alphabetical order.

The other:

Unless your change is a trivial or a documentation fix (e.g., a typo or reword of a small section) please:
Add yourself to AUTHORS;

It's not a big deal of course, I just noticed the difference.

[nicoddemus]

We can ignore the linting failure, it will be fixed in #2862 by a3ec3df.

[coveralls]

Coverage remained the same at 92.235% when pulling e23af00 on bilderbuchi:fix-1505 into 531e0dc on pytest-dev:master.