docs: add S2 provider docs

[yihuiliao]

Closes

✅ 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:

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation
• View the S2 docs

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation
• View the S2 docs

[reidbarber]

Are we wanting to include this?

[yihuiliao]

im fine to leave it out since it's covered by the styles guide but since we had a breakpoints section in the previous provider page for v3, people might be expecting it?

[snowystinger]

I think we can include it in the Migration guide and it should be covered by the style macros page as well