feat: add eleventy-plugin-footnotes for accessible footnotes

Registers the KittyGiraudel footnotes plugin and adds {% footnotes %}
to post.njk below the article content.

Usage in content:
  {% footnoteref "id" "Footnote text here" %}anchor text{% endfootnoteref %}

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -30,6 +30,7 @@
     "@zachleat/table-saw": "^1.0.7",
     "alpinejs": "^3.15.8",
     "eleventy-plugin-embed-everything": "^1.21.0",
+    "eleventy-plugin-footnotes": "^0.11.0",
     "gray-matter": "^4.0.3",
     "html-minifier-terser": "^7.0.0",
     "lite-youtube-embed": "^0.3.2",