Remove documentation about new remote caching in 2.19

[huonw]

This takes the unusual step of removing documentation. In particular, this removes the documentation for the still very experimental new remote cache backends: GitHub Actions and file system, because they're useless and very unstable:

• there's significant user-facing refactoring work that I'm not going to get to for 2.19 (Refactoring of remote stores after initial addition of OpenDAL #19902), particularly around changing how the options are configured
• the GHA backend seems to be very... limited, and only works for small repos (GitHub Actions Cache backend easily hits rate limit errors #20133)

I'm targeting 2.19, as it stabilises, and leaving the docs in main (for 2.20).