docs: move the prompt-caching answer into the published guide

The section landed in docs/TROUBLESHOOTING.md, a flat file removed in
a94e9493 when the docs were consolidated. Content there does not reach
the published guide, and the copy reintroduced install guidance that
develop had already pinned to --branch master.

Move the answer into docs/guide/resources/troubleshooting.md and drop the
duplicated Type Kit collision section, which that guide already covers.

The cache write/read breakdown is reported by `rtk cc-economics`, not by
`rtk gain`, so point readers at the command that actually shows it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>