Create a API Coverage Doc to track pyTFE API Coverage Completeness against Public Available Doc

[iam404]

Establish and maintain a single source of truth, docs/api-coverage.md that tracks pyTFE's API surface completeness against the publicly available HCP Terraform API documentation. The doc enumerates every public API resource, so users and contributors can see at a glance what's implemented, what's partial, and what's missing.