Add unified Description type for schema description handling

[typedrat]

Summary

• Introduces a Description module (lib/TerranixCodegen/Description.hs) that encapsulates description text + kind (plain/markdown), metadata enrichment, and rendering to hnix NExpr AST
• Replaces the internal buildDescription in OptionBuilder with Description.fromAttribute + Description.toNExpr
• Block descriptions now get deprecated metadata enrichment via Description.fromBlock
• Top-level resource/data source/provider descriptions now use the schema's actual block description when available, with fallback to the previous hardcoded strings
• Markdown-kind descriptions are wrapped in lib.mdDoc for proper rendering in NixOS module documentation

Closes three description generation gaps:

1. SchemaDescriptionKind was parsed but unused — now drives lib.mdDoc wrapping
2. Block descriptions had no metadata enrichment — now enriched with deprecation notes
3. Top-level module descriptions were hardcoded — now sourced from schema

Test plan

• 161 tests pass (23 new tests added across DescriptionSpec, OptionBuilderSpec, ModuleGeneratorSpec)
• nix flake check passes (build, format, lint)
• Existing tests pass unchanged (backward-compatible — plain descriptions produce identical output)
• New tests cover: fromAttribute/fromBlock metadata enrichment, markdown lib.mdDoc wrapping, top-level schema descriptions with fallbacks