Add documentation for advanced Go use cases.

[dsame]

Description:

1. Add the following documentation sections

• Multi-target builds
• Invalidate cache if source code changes
• Caching with actions/cache

2. Replace '-' with '##' in order to have link anchors in the document

Related issues: https://github.com/actions/setup-go/issues/316 https://github.com/actions/setup-go/issues/358 https://github.com/actions/setup-go/issues/395

Check list:

• [ ] Mark if documentation changes are required.
• [ ] Mark if tests were added or updated to cover the changes.

[fingon]

The monorepo instructions assume you build everything in every workflow too - if not, the cache is partially populated, and you would be better off having manual caching as opposed to what setup-go currently offers.