refactor(admonitions): admonition options breaking changes

[slorber]

Breaking change

• By default: admonition.keywords are appended to the default list of keywords instead of replacing the default list of keywords

Motivation

Let's use this PR to discuss / implement all the admonition API breaking changes we want to do.

Some pending questions:

• Do we want to give the ability to use custom tags per keyword? :::note, +++tip....
• Do we wait for siteConfig.markdown.admonitions config before merging this?

Test Plan

unit tests

Test links

Deploy preview: https://deploy-preview-8058--docusaurus-2.netlify.app/

Related issues/PRs

Follow-up of https://github.com/facebook/docusaurus/pull/7945

[netlify[bot]]

✅ [V2]

Name	Link
🔨 Latest commit	8ea187764334b82c9c14b165c0d88f74ad2e7e2b
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/6318bed2cd2d0a0008aac04d
😎 Deploy Preview	https://deploy-preview-8058--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[github-actions[bot]]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 80	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 72	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions[bot]]

Size Change: 0 B

Total Size: 813 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.1 kB
website/build/assets/css/styles.********.css	111 kB
website/build/assets/js/main.********.js	609 kB
website/build/index.html	40.8 kB

_{compressed-size-action}

[slorber]

Changes have been integrated in the MDX 2 PR: https://github.com/facebook/docusaurus/pull/8288