DOC/WEB: Move not versioned docs to the web

The reason we have a separate website and docs is that the docs are specific to a version (e.g. pandas 1.5.3), while the website is global to all versions and updated in real time.

For historic reasons, we have several things that are not version specific in the docs, which IMHO doesn't add any value and brings confusion and extra work backporting and keeping things sync. To be specific, what's in the docs that is not version dependent is:

• Development documentation
• Release notes
• Installation instructions (just a small section in the Getting started docs)

The change is not trivial for two reasons:

• The docs use restructured text, and the website uses markdown
• The website doesn't provide automatic references (we can't use things like :meth:'Series.isin')

For the development documentation I think the second point is not a big problem, but for the release notes we do use references to the API a lot. Not sure if it's worth the migration, but creating an issue anyway for visibility of the problem, and in case anyone has comments.