Moved documentation to docs directory

[svengregorio]

Fixes #1479

• I have added a news fragment under changelog.d/ (if the patch affects the end users)

Summary of changes

Moved repository documentation in the docs directory, so that both GitHub's markdown viewer and the official documentation can follow links in the markdown documentation.

Test plan

Tested by clicking on the repaired link in the Use with pre-commit section

[svengregorio]

Apparently fixing the link for Github, breaks it for nox. This may need to become an issue instead of a PR.

[uranusjr]

The link target is a symlink, problem is GitHub’s code viewer does not support that…

[dukecat0]

Or perhaps we can simply change it to https://pipx.pypa.io/stable/installation/#using-pipx-with-pre-commit?

[Gitznik]

Or perhaps we can simply change it to https://pipx.pypa.io/stable/installation/#using-pipx-with-pre-commit?

This would be very confusing when looking at the docs on not stable. E.g. clicking the link on the latest docs would redirect you to the stable docs.

[svengregorio]

According to GitHub's README documentation, README.md could be in the docs directory, eliminating the need for symlinks. I could test whether this approach truly works in the next days.

[svengregorio]

I retroactively opened issue #1479 for this PR, I hope this doesn't cause any confusion.