feat(schema): autogenerate manifest reference docs

[31puneet]

Description

Adds a Python generator script (schema/generate_manifest_docs.py) that introspects the Pydantic models in model.py to auto-generate structured Markdown reference documentation for pixi.toml. Mirrors the existing CLI auto-generation pattern and includes CI checks to ensure documentation stays up to date.

Fixes #3021

How Has This Been Tested?

• Ran pixi run -e schema generate-manifest-docs to generate the reference document.
• Ran pixi run -e docs build-docs (mkdocs build --strict) to verify navigation and formatting.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[baszalmstra]

Thanks for working on this! This is only part of the solution. The goal is to use the schema as a baseline and adapt it further with custom content to add more explanation, examples, etc. If we were to merge this, we'd have two separate pieces of documentation:

• A generated one with very little information, not very human-friendly.
• A potentially outdated, manually written documentation.

That doesn't add much value.

[31puneet]

Thanks for the feedback! Makes sense, I'll rework this so we have a single, cohesive documentation page that uses the schema as the baseline structure and injects the manual examples and explanations into it.

[baszalmstra]

Or if we can add richer information to the schema itself that would also be good! Examples and such. Not sure how much of this is possible with json schema though.

[31puneet]

Yeah, enriching model.py is a good idea since it improves both the docs and the JSON schema for IDE users. Pydantic's Field() already supports examples and many fields already use it, so we can build on that and make the generated output richer.

[31puneet]

PTAL @baszalmstra Thanks!