Move installation.rst to installation/*.rst

[aclark4life]

• Add index
• Move content to sections
• Rearrange sections

Changes proposed in this pull request:

• Adds navigation to Installation section
• Rearrange sections to prioritize basic installation, followed by supported platforms and Python versions, followed by building from source and old releases

[hugovk]

My biggest concern is linkrot, existing links out will break and result in 404s.

For example, answers on Stack Overflow will no longer link to the correct place.

[aclark4life]

@hugovk Good call, I've added a95ca25 to address that issue.

[hugovk]

Is there a way to redirect links like this?

https://pillow.readthedocs.io/en/stable/installation.html#building-from-source

With the preview:

https://pillow--7832.org.readthedocs.build/en/7832/installation.html#building-from-source

It now only goes to the ToC:

https://pillow--7832.org.readthedocs.build/en/7832/installation/index.html

[aclark4life]

@hugovk Hmmm. Possibly if we add redirects on RTD according to pypa/packaging.python.org#1401 (comment) (with the core issue being whether or not link fragments are handled by the HTTP specification.) At that point, the discussion becomes more about weighing the benefit of adding navigation links to the installation section sidebar vs. breaking SO links, or SEO or whatever. I'm pretty happy with adding sphinx-reredirect and configuring RTD redirects as needed (assuming that works as expected, which I'm not inclined to believe it will and also feels a bit "heavy") and calling it a day, but that's just me. 🤷

[aclark4life]

Alternatively, we could leave note, something like:

[aclark4life]

@hugovk @radarhere OK to merge this one? 🤔

[aclark4life]

@hugovk @radarhere OK to merge this one? 🤔

Hold on to your butts

[hugovk]

Sorry for the late reply!

I think this is mostly fine, the original page was very large and it definitely looks much better for most of these to be their own page. 👍

I've opened #7858 with some little tweaks.