document and enable caching of incremental build artifacts

**Context**

To speed up builds, unity uses an incremental build pipeline. Caching the files produced by this pipeline has the potential to speed up the build process dramatically.

**Suggested solution**

Document where the incremental build cache is stored so people can cache that folder.
One issue is that I was not able to find the incremental cache folder on windows runners (needed for windows IL2CPP builds), so the suggested solution would be to add an option to specify the incremental build artifact directory in the action parameters (adding a new parameter for it).

**Considered alternatives**
🤷

**Additional details**

For unchanged code, even on IL2CPP, the build takes less than 5 minutes, but I've seen 2x improvement in build time on mono build even if there are code changed.