Document fixes / workarounds for rust-analyzer breaking incremental recompiles

A user [ran into this problem](https://discord.com/channels/691052431525675048/1167175577233539113) after following the linked advice on fast compiles.

I'm not entirely sure on the best solution.

We could:

1. Suggest that they change the target directory for rust-analyzer, using https://github.com/rust-lang/rust-analyzer/pull/15681
2. Suggest they use [sccache](https://github.com/mozilla/sccache) to share build artifacts (very useful when playing with many Bevy projects).
3. Use the workarounds listed [here](https://github.com/rust-lang/rust-analyzer/issues/4616#issuecomment-633852029).
4. Do something else that I haven't thought of! I'm not a compiler/build tools wizard 😅