refactor(theme): split admonitions, make swizzle easier, better retrocompatibility

[slorber]

Motivation

• It should be easier to customize the layout/view of an admonition
• it should be easier to add a custom admonition type
• :::important should display label "important" (as it used to be), fix "Important" admonition has "info" as label #7767

Test Plan

preview

Test links

Deploy preview:

• https://deploy-preview-7945--docusaurus-2.netlify.app/docs/markdown-features/admonitions
• https://deploy-preview-7945--docusaurus-2.netlify.app/tests/pages/markdownPageTests

Local:

• http://localhost:3000/docs/markdown-features/admonitions
• http://localhost:3000/tests/pages/markdownPageTests

[netlify]

✅ [V2]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 82	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 85	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: +48 B (0%)

Total Size: 813 kB

Filename	Size	Change
website/build/assets/js/main.********.js	609 kB	+48 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.1 kB
website/build/assets/css/styles.********.css	111 kB
website/build/index.html	40.8 kB

_{compressed-size-action}

[Josh-Cena]

Maybe add some docs about this? (Wasn't satisfied with our admonition customization docs)

[slorber]

Yes, it's not finished yet.

I'm also trying to see what can be shared, so that it's easier later to add a Tailwind theme without too much duplication.

I guess we'll also want to share things such as the default labels and icons which are not really theme-specific, but we still want the ability to swizzle to override? 🤷‍♂️

[slorber]

Ready for review.

Satisfied with the current implementation: will merge soon unless I get feedback

[Josh-Cena]

I'll try reviewing this tonight.

[slorber]

Looks good enough: marked as safe to swizzle.

Note: as we have more "mapping objects" in themes, we should probably add a way to declare a theme module is an object. This way we could support swizzle --wrap such objects. Showing "wrap is forbidden" to the user is a bit confusing considering we encourage wrapping (manually) in our doc

We'll probably do some more admonition options breaking changes in the future but probably not before v4.0. It looks good enough for me to commit to respecting semver