Add namespace documentation for all Terminal.Gui.Editor namespaces

[tig]

Problem

The Terminal.Gui.Editor namespaces lack namespace-level XML documentation that provides an overview of each namespace's purpose and contents.

Expected

Each Terminal.Gui.Editor.* namespace should have namespace docs (via a NamespaceDoc class or equivalent mechanism) similar to how the Terminal.Gui library documents its namespaces. These show up in the docfx-generated API reference as the namespace summary.

These docs should leverage what's already written in README.md.

The main Editor namespace should show the Terminal.Gui.Editor repo's README hero.gif.

Reference

See how Terminal.Gui does it — each namespace has a summary that appears in the API docs TOC and namespace landing pages.