[WIP] docs(S2): add Custom Components docs and Examples page

[reidbarber]

Adds a new "Custom Components" page to the S2 docs and an "Examples" page, similar to how we have for React Aria.

I'm still on the fence about the content, since there is a decent amount of overlap with the existing Styling docs, but it seems like we need better guidance towards combining React Aria Components + the S2 Style macro for creating custom S2 components.

Also not tied to the current Button examples, since they may be too similar to our existing Button.

We'll also need to think through what makes good "examples" and doesn't get too close to existing components.

Todo:

• Add more "best practices"
• Prop types (Extend RAC Props)
• Examples
• Animations? (For popover, modal, etc.)
• pressScale? (Should we export?)
• Arbitrary value syntax? (Maybe put this and css under an "Escape Hatches" section)
• Other utilities?
• Interactive demos (similar to S2 home page)

✅ 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 S2 docs
• View the React Aria docs

[rspbot]

Build successful! 🎉

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

[rspbot]

Build successful! 🎉

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

[rspbot]

Build successful! 🎉

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