Document use of tests for resourcedocsgen tool

[guineveresaenger]

https://github.com/pulumi/registry/pull/5931 added tests for resourcedocgen. 🎉

For the developer, the setup of these tests includes some context that is not immediately obvious.

Since we do not have a standardized way to do tests, it's really important that tests have:

• Setup instructions
• Documentation on what to avoid when writing new tests
• How to re-generate the golden files