✨ feat(shortcodes): add admonition shortcode

[welpo]

Summary

Adds "admonition" shortcode, inspired by Docusaurus.

Using them is as simple as:

{{ admonition(type="note", title="Custom title", text="Some **content** with _Markdown_ `syntax`. Check [this `api`](#).") }}

Docs available in the shortcodes page.

Screenshots

Type of change

• Bug fix (fixes an issue without altering functionality)
• New feature (adds non-breaking functionality)
• Breaking change (alters existing functionality)
• UI/UX improvement (enhances user interface without altering functionality)
• Refactor (improves code quality without altering functionality)
• Documentation update
• Other (please describe below)

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Checklist

• I have verified the accessibility of my changes
• I have tested all possible scenarios for this change
• I have updated theme.toml with a sane default for the feature
• I have made corresponding changes to the documentation:
  • Updated config.toml comments
  • Updated theme.toml comments
  • Updated "Mastering tabi" post in English
  • (Optional) Updated "Mastering tabi" post in Spanish
  • (Optional) Updated "Mastering tabi" post in Catalan