feat: Add AdmonitionExtension for semantic admonition markup

[dereuromark]

Summary

Adds a new AdmonitionExtension that transforms divs with admonition type classes into semantic HTML. Inspired by Python-Markdown/Material for MkDocs admonition syntax, but using djot's native fenced div syntax.

Features

• Recognizes admonition types: note, tip, warning, danger, info, success, caution
• Adds proper ARIA roles (role="note" or role="alert" for warning types)
• Auto-generates titles from type name (configurable)
• Supports custom titles via {title="Custom Title"} attribute
• Supports collapsible blocks via {collapsible} or {collapsible=open} attributes
• Fully configurable (types, title tag/class, container class)

Usage

$converter = new DjotConverter();
$converter->addExtension(new AdmonitionExtension());

Input/Output Examples

Basic admonition:

::: note
This is a note.
:::

<div class="admonition note" role="note">
  <p class="admonition-title">Note</p>
  <p>This is a note.</p>
</div>

With custom title:

{title="Watch Out!"}
::: warning
Be careful here.
:::

Collapsible:

{collapsible}
::: tip
Click to expand.
:::

<details class="admonition tip">
  <summary>Tip</summary>
  <p>Click to expand.</p>
</details>

[codecov]

Codecov Report

❌ Patch coverage is 95.38462% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.97%. Comparing base (41bdb80) to head (11d2c08).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Extension/AdmonitionExtension.php	95.38%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #129      +/-   ##
============================================
+ Coverage     93.94%   93.97%   +0.02%     
- Complexity     2385     2419      +34     
============================================
  Files            81       83       +2     
  Lines          6310     6403      +93     
============================================
+ Hits           5928     6017      +89     
- Misses          382      386       +4     

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