Fix/928 autoformatting docs

[aaroncymor]

What

Integrates blacken-docs to auto-format Python code blocks in Markdown documentation while preserving intentional Air tag nesting patterns using <!-- blacken-docs:off/on --> guards.

Fixes issue #928

Pattern

Follows the existing formatting workflow established in justfile.

Reviewer Focus

• Guard placement: Verify that <!-- blacken-docs:off/on --> guards are only around Air tag code blocks with intentional multi-line nesting (e.g., air.Html(), air.layouts.mvpcss() with nested children), not around plain Python code.
• Line length consistency: Confirm --line-length 120 matches project's Ruff configuration.

Checklist

• Diff contains only changes for this task — no unrelated refactoring or cleanup
• Addresses exactly one issue or feature
• New or changed behavior has test coverage (N/A - formatting tool integration)
• This is the simplest viable approach
• AI provenance section removed or accurate