[Docs] Add docs for Organization Settings

[seanpoulter-foci]

Add general documentation for how to manage organization settings using GitHub Foundations similar to how we're already doing Teams, Repos, Secrets, etc.

This corresponds to removing the import block in FociSolutions/github-foundations-modules#56:

-import {
-  to = github_organization_settings.organization_settings
-  id = var.github_organization_id
-}