Support manual class documentation in the documentation build

[MariusStorhaug]

Context

PowerShell modules built with the PSModule framework can have public classes, but there is no established convention for where to place manually written class documentation. While auto-generation from AST is a separate initiative (#101), some classes may need hand-authored documentation.

Request

The documentation framework should support a classes/ folder (or similar convention) where developers can place manually created markdown files for class documentation. These files should be picked up by the documentation build and included in the generated site.

Acceptance criteria

• A convention is established for where to place class documentation files
• Manually created class documentation files are included in the documentation build
• Class documentation appears in the site navigation

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Technical decisions

To be determined during implementation planning — evaluate whether to use a docs/classes/ folder or co-locate documentation with class source files.

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Implementation plan

• Define the convention for class documentation file placement
• Update the documentation build to discover and include class docs
• Add class documentation to the MkDocs navigation structure
• Document the convention for module developers

[MariusStorhaug]

Restructured issue description into the standard three-section format.

• Rewrote into a proper context/request structure
• Updated title to imperative mood (removed emoji prefix)
• Created implementation plan with 4 tasks
• Applied Minor label (new feature)