✨ feat(admonitions): allow swapping admonition icons

[welpo]

Summary

This PR adds the ability to customise icons for admonitions.

Related issue

Resolves #337.

Changes

• Updated admonition shortcode to accept a custom icon parameter
• Modified CSS to support custom icons while maintaining existing styles
• Added documentation for the new icon customization feature

Accessibility

The changes maintain the existing accessibility features of admonitions.

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

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