docs: improve S2 docs markdown / agent skill

[reidbarber]

Includes improvements to the markdown docs output and S2 agent skills.

Markdown output improvements:

• skip redirect pages (the mcp.mdx pages got replaced by the ai.mdx pages)
• skip error pages (no need for errror.md files)
• fix inline code being stripped from page descriptions (visible here)
• include Routers section in S2 Getting Started page (render at bottom of page)
• improve Style Macro docs (more clear and concise, uses aliases for repetitive sets)

Skill Improvements:

• fix heading levels in component-decision-tree.md
• improve illustration example in implementation-guidance.md to include all three types
• improve skill with guidance using Buttons with icons + text (ensure text is wrapped in Text)
• improve skill with guidance to avoid concatenating class names from the style macro

General docs improvements

• add 'dnd' tag for RAC dnd docs page (so it shows up if you search 'dnd' in the docs)
• mention passing isFocusVisible to focusRing in JSDoc for focusRing

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

View markdown docs output and skill output for affected files.

To see the agent skill diff, first make sure you have the latest release's skill installed:

npx skills add https://react-spectrum.adobe.com --skill react-spectrum-s2

then install this PR's and look at the diff:

npx skills add https://d1pzu54gtk2aed.cloudfront.net/pr/91cb93c5f08636a568323bf91c05c969ebba5bd0/ --skill react-spectrum-s2

🧢 Your Project: