List every single backend in docs (Cherry-pick of #19854, #19880) (#1…

…9878)

This adjusts the https://www.pantsbuild.org/docs/enabling-backends
backends page to list every single backend, as implied by `find . -name
register.py`. This makes the tables much larger, so I've split it into
stable vs experimental.

Why do this? Aren't experimental backends 'dangerous' for people to try?
... If people don't know the backends exist, they won't try them/file
bugs/submit improvements, and they'll never be stabilised.

I've done a very basic pass on filling out the content, without trying
to scope creep into writing new docs for things, but, having the table
makes it obvious which things likely could have docs improved: if
there's no link at all! There's various low hanging ways we could
improve this (e.g. add more backends to `generate_docs.py` to at least
have a subsystem or goal reference page to link to), as well as
higher-hanging ways (write whole new doc pages).

The commits are individually reviewable.

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Co-authored-by: Huon Wilson <huon@exoflare.io>