Documentation restructuring

[evildmp]

A complete restructuring of pytest's documentation in line with https://diataxis.fr. See pytest-dev email list discussion.

Progress so far

• Renamed Install to Getting started; moved notes to index
• Reworded titles of some clear how-to guides
• Moved documentation topics to group them by theme
• Added how-to and reference directories
• Split fixtures documentation into separate documents
• Created a new Explanation section and new landing pages for main sections
• Added myself to AUTHORS
• Moved more sections from reference to how-to
• Restructured global TOC
• Explicitly listed the four documentation sections on index.rst
• Restructured 'How to invoke pytest' guide
• Further rationalised "how to invoke pytest" material

[bluetech]

I noticed that since we are moving pages around, old links break, like https://docs.pytest.org/en/latest/fixture.html and https://docs.pytest.org/en/latest/reference.html, which I figure is not great because they have a lot of history. I think we'll want to have some solution for this, maybe keeping "page moved" pages, or using something like sphinx-redirects (probably better). I'll try to set it up if no one beats me to it.

[evildmp]

Redirects is better. Also, at some point I would like to change the builder so that pages lose the .html - which I think is more elegant.

[bluetech]

Created a new issue regarding the redirects: #8471