[docs] Standardize the usage of callouts in the MUI Core docs

[samuelsycamore]

This PR is part of the ongoing effort to standardize the style and formatting of the MUI docs.

The purpose here is to enforce consistent usage of the four types of callouts, based on the rules set forth in the doc I've created in the Company Handbook.

Additionally, I've removed all signifiers (⚠️, 💡, Note:, Tip:, etc.) within the callouts, which are redundant now that we have different types.

I've also made some tiny grammar and style changes in and around the callouts where I noticed them, but tried to limit the scope of this PR by not actively editing the content.

If/when we're in agreement about the implementation of the style rules here, I'll perform this same audit for the X and Toolpad docs.

https://deploy-preview-35361--material-ui.netlify.app/experiments/docs/callouts/

[mui-bot]

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[oliviertassinari]

Additionally, I've removed all signifiers (⚠️, 💡, Note:, Tip:, etc.) within the callouts, which are redundant now that we have different types.

@samuelsycamore It sounds great, we can later implement them in a systematic fashion based on the design direction that we would like to them to look like 👍

• Stripe https://stripe.com/docs/billing/subscriptions/overview
• Docusorus https://docusaurus.io/docs/docusaurus-core#errorboundary
• Us https://deploy-preview-35361--material-ui.netlify.app/experiments/docs/callouts/
• Nextra https://nextra.site/docs/docs-theme/built-ins/callout#default
• Minimist https://mintlify.com/docs/components/callout
• Tailwind CSS ❌ can't find any, they don't use this pattern. Maybe we should use it parsimoniously.

[danilo-leal]

Looking great 🙌 Thanks for setting up the experiments page too, though ⎯ I have a branch open for doing some design tweaks to the callouts and that will be useful. Hoping we do add some icons "natively" later on, as suggested above, to drive more attention and to differentiate them a bit on the UI (a container with text only feels a bit empty). cc @siriwatknp