Expand documentation on contributing new resources

[melinath]

Contributing to terraform-validator is complex and in some cases requires modifying Magic Modules. We should more closely document the process. In particular, we should:

• document how to modify handwritten files & reference them in compile_common_files and copy_common_files
• document how to add IAM support specifically, including descriptions of what the merge and fetch functions do
• document how to add tests in more detail

[melinath]

IAM docs should potentially explicitly mention providing project unless #206 (comment) has been resolved, since IAM often will not have a project field.

[melinath]

I added some more detail for adding tests as part of #333 but maybe we want to do more.

[marktru]

Internal issue: b/211902384