chore: ui overview docs

[Chriscbr]

Add a dedicated page with an introduction to the ui module:

Checklist

• Title matches Winglang's style guide
• Description explains motivation and solution
• Tests added (always)
• Docs updated (only required for features)
• Added pr/e2e-full label if this feature requires end-to-end testing

By submitting this pull request, I confirm that my contribution is made under the terms of the Wing Cloud Contribution License.

[github-actions]

Thanks for opening this pull request! 🎉
Please consult the contributing guidelines for details on how to contribute to this project.
If you need any assistance, don't hesitate to ping the relevant owner over Discord.

Topic	Owner
Wing SDK and standard library	@chriscbr
Wing Console	@skyrpex
Wing compiler and language design	@chriscbr
VSCode extension and language server	@chriscbr
Wing CLI	@chriscbr
Documentation	@boyney123
Examples	@boyney123
Wing Playground	@skyrpex

[boyney123]

Nice one mate!

[mergify]

Thanks for contributing, @Chriscbr! This PR will now be added to the merge queue, or immediately merged if rybickic/ui-docs is up-to-date with main and the queue is empty.

[monadabot]

Congrats! 🚀 This was released in Wing 0.85.4.