Improve documentation UX for sync and async APIs

[python-trio]

Improve documentation UX for sync and async APIs

[pquentin]

What do you think about using something like https://djungelorm.github.io/sphinx-tabs/#code-tabs?

When you choose a different language, all other snippets in the page automatically show that language.

This specific extension does not degrade gracefully: without JS we only see the first tab instead of seeing all of them. I guess that's fixable. But at least that's a solution that already works today.

Should this slider be extended to Sync, Trio, Asyncio, Curio?

I guess ultimately including all frameworks we support is nice because I prefer code snippets that can be run as-is. It shouldn't be too hard as everything should be generated anyway. But it's not mandatory either, just a nice touch.

[sethmlarson]

Maybe the tabbed approach can be used for the asynclib-specific examples and the global Sync / Async can apply to the API ref