Add "Why does Rust Analyzer use so much RAM and CPU?" to manual

[SamB]

#11325 might not cover everything that should be in such a chapter/section, but has an excellent title at least.

For content, I'd want to see information about what features tend to eat lots of RAM or CPU, how to measure it (if rust-analyzer doesn't die immediately), and suggestions on how to deal with this (disabling features, tuning limits, rearranging code, reducing macro output size, attempting to redesign aspects of rust-analyzer, or any other thing).

[ChayimFriedman2]

Unfortunately, I don't think we have a lot to say in this area. The best we can say is "did you try disabling cache priming?" and otherwise "wait for r-a to improve (or contribute it yourself)".