docs(cache): add more information on the different cache(s) and what gets used where

[jamietanna]

We currently have (at least?) 2 types of cache for Renovate:

• repository cache for extraction results
  • repositoryCache and related variables control this
• "global" cache for "what available updates do we have for ...?"
  • redisUrl and related options

We should provide a bit more in-depth documentation about how these actually work, what is part of them, and what a recommended topology may be.

Relates to #40426.

[jamietanna]

@zharinov I'm happy picking this up, but may need a hand digging into some of the details that you know much better than me!

(This is one we can do after #40426)

[zharinov]

Yeah, working on this one too