Add DefaultFunc to repo ruleset required deploy env, base role IDs to docs

[o-sama]

Resolves #1915

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Before the change?

• If an empty list is passed to rules.required_deployments.required_deployment_environments, the following is what we get:
  required_deployments:[<nil>]
  The reason it's bad is because we want to be able to strictly specify an empty list, the GitHub API accepts that as a valid input.
  The reason it's set to nil is because the elements of rules.required_deployments are pointers, an empty slice is the zero value and thus treated as a nil pointer.

After the change?

• Setting a DefaultFunc to return an empty slice gives us the following:
  required_deployments:[map[required_deployment_environments:[]]]
  The API accepts this as a valid input and can successfully apply changes. It also doesn't affect anything that's existing since any input overrides the DefaultFunc.
• Added base role mapping in the docs since there seems to be confusion regarding this.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[nickfloyd]

Thanks for catching all of this @o-sama 💥

[o-sama]

Thanks man! I'm just fixing things people are finding, couldn't do it without all the issues being raised 🙂