docs: improve documentation for input/output types

[markphelps]

Summary

• Expands docs/python.md input/output type documentation into a complete reference with examples and limitations
• Regenerates docs/llms.txt
• Adds repository-wide Markdown linting via markdownlint-cli2 in mise and CI

Fixes #2854

Documentation updates (docs/python.md)

• Rewrites the Input and output types section from a short bullet list into structured reference docs
• Adds a primitive type table with JSON Schema mappings
• Documents wrapper/container usage and examples for:
  • Optional[T] and T | None
  • list[T] (input and output)
  • dict (opaque JSON object)
• Adds structured output coverage for:
  • cog.BaseModel
  • Pydantic v2 BaseModel
  • supported BaseModel field types
• Adds explicit type limitations (e.g. nested generics and unsupported unions)
• Corrects prior inaccuracies:
  • AsyncIterator should be imported from typing (not cog)
  • output model class is not required to be named Output
  • clarifies os.PathLike compatibility notes

Markdown linting/tooling updates

• Adds markdownlint-cli2 as a mise-managed tool (npm:markdownlint-cli2)
• Adds .markdownlint-cli2.yaml with scoped baseline rules and excludes
• Adds/updates mise tasks:
  • lint:docs, lint:docs:fix
  • fmt:docs, fmt:docs:fix now run markdownlint-cli2
  • includes docs linting in aggregate lint and lint:fix
• Updates CI workflow to:
  • detect docs changes for any *.md file
  • run a dedicated lint-docs job
  • include lint-docs in the final ci-complete gate
• Applies small markdown autofixes (trailing whitespace) in existing markdown files touched by --fix