[core] Batch small changes

[oliviertassinari]

• [docs] Prefer export default ccd5aee: the large majority of the codebase use this approach. I suspect we have migrated 100% after these changes.
• [docs] Experiment with a different price point 0d0f91e: the pricing point comes from https://sidekiq.org/support.html. After hearing his podcast, I suspect it's not something he is interested in doing a lot, hence put a deterrent pricing. I would like to try something more reasonable.
• [docs] Add a new backer 6bf822a: I have forgotten to add some in the past as well to remove the ones that stop contributing to Patreon. I don't feel productive when I update this list!
• [test] Remove redundant cleanup 306a7dc: I believe I have removed duplicated logic. cc @eps1lon for confirmation
• [docs] Simpler copy & paste of the global class names c8e2a46: I think that the dot prefix makes is simpler to understand that it's a CSS class name.
• [docs] Fix facebook icon synonyms ddece9b: I have found this one by luck.
• [docs] Improve the Popper demos to be more selling 89135f2: The more focused the demo is, the better.
• [docs] Write down the imperative API benefit 5bf75de: Also use the success variant that has a better color contrast. https://dev.to/dmtrkovalenko/the-neatest-way-to-handle-alert-dialogs-in-react-1aoe is pretty cool in this imperative API landscape by @dmtrKovalenko.
• [docs] Improve the Style Library Interoperability 1ac0c0d: help with Unified React theming with other CSS-in-JS libraries? emotion-js/emotion#998 (comment) and classes prop not working with emotion css. #17199 (comment). I have removed Glamor as the npm downloads are going down and fast.
• [docs] Improve blog post wording c991be6.
• [docs] Fix modal markdown indent b7173ca.
• [core] Avoid deep imports 8d52a1e. We were mixing the two approaches. cc @eps1lon
• [TreeView] Yes, it's accessible :) 1c9d67b: We might as well say it :) cc @joshwooding
• [docs] Add children when supported 619fc47: closes [Docs/API] Some components missing children prop #17674

A 2 hours effort. I might have bundled too many unrelated topics 😔. After ~8 commits, it feels too much.

[mui-pr-bot]

No bundle size changes comparing 5d564f9...133e635

Generated by 🚫 dangerJS against 133e635

[eps1lon]

Looks good. Though 2400 line diff is barely manageable in GitHubs UI.

[oliviertassinari]

Looks good. Though 2400 line diff is barely manageable in GitHubs UI.

@eps1lon Agree! Thanks for the review, the API markdown diff is blowing it way.

Starting of with parentheses looks weird. I would just say that it implements the behavior described in the aria practices and include the link. We don't know if it is "complete" since that isn't defined and "no assistance required" might be misunderstood. You still need assistive technology to use it.

I'm eager to find a better approach to link the relevant accessibility resources. The parentheses are used in the other pages. I hope we can find a solution that also enables us to link

1. The source, very important to increase people's trust when they see, for instance, a TextareaAutosize: where are the sources?!
2. The material design specification. It's relatively random right now.
3. The relevant accessibility resources.

I have started to benchmark a bit how we could solve this problem (for a potential future pull request)

https://vuetifyjs.com/en/components/buttons#buttons

https://orbit.kiwi/components/collapse/react/

[mbrookes]

Suggested change

	We demonstrate how to use [notistack](https://github.com/iamhosseindhv/notistack).
	This example demonstrates how to use [notistack](https://github.com/iamhosseindhv/notistack).

[mbrookes]

Suggested change

	notistack has an **imperative API** that makes it easy to display toasts (so you don't have to deal with open/close state of them).
	notistack has an **imperative API** that makes it easy to display snackbars, without having to handle their open/close state.

[mbrookes]

Oops! Started this this morning and missed that it was since merged.