fix(cli): omit internal interfaces in generate-doc output

[josefpihrt]

Summary

• Filter implemented interfaces in generate-doc with IsPubliclyVisible() so internal (and otherwise non-public) interfaces are omitted from consumer-facing documentation
• Applies to type declaration snippets, the Implements section, and member-level Implements annotations
• GetDisplayParts takes an optional shouldDisplayInterface callback so list-symbols can still honor its own visibility filter
• Fixes public class Public : IInternal producing links to internal IInternal in github and docusaurus output

Fixes #1635

Test plan

• dotnet build src/Documentation/Documentation.csproj
• dotnet build src/CommandLine/CommandLine.csproj

[josefpihrt]

Review notes for omitting restrictive interfaces from generate-doc. The accessibility filter matches #1635; a couple of cleanup/test notes.