feat(docs): add markup primitives render

[mikhd]

[3] Add the default markdown MarkupRenderer and the renderType type-cell renderer with its docs-local child classification on top of the #53 . No page assembly yet.

To avoid relying on external libs and due to simplicity, markdown code is currently hand-written (except for markdown-table). Please let me know if we would wanted to use any of the markdown libs instead.

Adds:

• markdownRenderer (render/markdown.ts) - the default MarkupRenderer. Blocks, headings, paragraphs, code spans/blocks, links, bold/italic, ordered/unordered lists, and column-padded tables.
• renderType (render/renderType.ts) - renders one type-cell from a TypeExpr: entity refs become code-wrapped links (TypeNode), primitives.
• Child classification - isDocChild decides whether an attribute goes under a page's Data or Children section.

[mikhd]

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• feat(docs): add examples #57
• feat(docs): prepare @codama/spec/docs package, add doc .md files #56
• feat(docs): render all page types and wire generateDocs #55
• feat(docs): add markup primitives render #54 👈 (View in Graphite)
• feat(docs): add nav registry #53
• feat(docs): add doc-model types and helpers #52
• main

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