Autodoc: missing integration with the cache system #15864
Description
Today autodoc just happily places all its artifacts at the destination and calls it a day.
If you run zig test foo.zig -femit-docs, the cache system should be able to notice that the tests don't need to be re-run (it already does that), but that the doc files in the destination folder (docs/ by default) need to be replaced with a cached copy in case they're missing / stale.
Right now users will experience intermittent malfunctioning if they try to generate autodocs from zig test and other similarly cacheable commands.
This requires integrating autodoc correctly with the compiler caching system.