chore: speed up markdown docs script

[reidbarber]

The markdown docs generation script (yarn workspace @react-spectrum/s2-docs generate:md) was pretty slow due to repetitive sync glob and file reads. Now, we build an index of symbols and read from that instead.

On my machine:

Before: 113s
After: 5s

✅ 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:

Smoke test that markdown docs outputs are unchanged.

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs