Add admonitions plus autolink and sub/superscript markdown extensions

!!! note/warning/tip/... blocks (mdit-py-plugins admon) render as
lightweight callouts: accent bar + faint wash in the blockquote idiom,
recolored per type (tip green, warning pink/red via --admonition-color),
rounded and softly shadowed in the summer theme. Also enabled
gfm_autolink (bare URLs) and H~2~O / x^2^ sub/superscripts.

AGENTS.md

@@ -91,7 +91,8 @@ not for the public pages. See `docs/design-principles.md` for the design.
     on every page, otherwise browsers fall back to the build's
     `/favicon.ico` by convention.
   - `markdown.py` — markdown-it-py renderer (html passthrough + attrs,
-    footnote, deflist, tasklists plugins; typographer + breaks on). Custom
+    footnote, deflist, tasklists, admon, gfm_autolink, sub/superscript
+    plugins; typographer + breaks on). Custom
     image rule: relative srcs resolve against the page path; an image
     standing alone in its paragraph becomes a figure (captioned when
     titled), while inline-with-text images and raw <img> HTML stay plain.

frontend/src/assets/pagerite.css

@@ -541,6 +541,42 @@ blockquote p {
   margin: 0;
 }
 
+/* Admonitions (markdown !!! note/warning/...): a lightweight callout in
+   the blockquote idiom — accent bar and a faint wash, recolored per type.
+   Bottom-only margins like everything else in articles; inner paragraphs
+   carry no margins of their own. */
+.admonition {
+  margin: 0 0 1rem;
+  padding: 0.4rem 0.9rem;
+  border-left: 0.25rem solid var(--admonition-color, var(--accent));
+  border-radius: 0 0.3rem 0.3rem 0;
+  background: color-mix(in srgb, var(--admonition-color, var(--accent)) 7%, transparent);
+}
+
+.admonition > :last-child {
+  margin-bottom: 0;
+}
+
+.admonition-title {
+  margin: 0 0 0.2rem;
+  font-weight: 600;
+  color: var(--admonition-color, var(--accent));
+}
+
+.admonition.tip,
+.admonition.important,
+.admonition.hint,
+.admonition.success {
+  --admonition-color: var(--accent2);
+}
+
+.admonition.warning,
+.admonition.caution,
+.admonition.danger,
+.admonition.failure {
+  --admonition-color: var(--accent3);
+}
+
 pre {
   overflow-x: auto;
   padding: 0.5rem 0.8rem;

pagerite/markdown.py

@@ -2,9 +2,11 @@
 
 Raw HTML (including inline scripts) is passed through unfiltered: the
 single author is trusted. Extensions: tables and strikethrough (from the
-"default" preset), footnotes, definition lists, task lists and
+"default" preset), footnotes, definition lists, task lists,
 brace-attributes (`{.class width=300}` on any element, images in
-particular).
+particular) and admonitions (``!!! note Title`` with an indented body —
+note/tip/warning/etc., the title optional). Bare URLs autolink (GFM), and
+``H~2~O`` / ``x^2^`` give sub/superscripts.
 
 markdown-it's typographer is enabled, so body text gets SmartyPants-style
 replacements: straight quotes become curly, ``--`` / ``---`` become en / em
@@ -27,9 +29,13 @@ from datetime import datetime, timedelta
 from markdown_it import MarkdownIt
 from markdown_it.common.utils import escapeHtml
 from markdown_it.renderer import RendererHTML
+from mdit_py_plugins.admon import admon_plugin
 from mdit_py_plugins.attrs import attrs_plugin
 from mdit_py_plugins.deflist import deflist_plugin
 from mdit_py_plugins.footnote import footnote_plugin
+from mdit_py_plugins.gfm_autolink import gfm_autolink_plugin
+from mdit_py_plugins.subscript import sub_plugin
+from mdit_py_plugins.superscript import superscript_plugin
 from mdit_py_plugins.tasklists import tasklists_plugin
 from pygments import highlight
 from pygments.formatters import HtmlFormatter
@@ -135,9 +141,13 @@ md = (
         },
     )
     .use(attrs_plugin)
+    .use(admon_plugin)
     .use(footnote_plugin)
     .use(deflist_plugin)
     .use(tasklists_plugin, enabled=True)
+    .use(gfm_autolink_plugin)
+    .use(sub_plugin)
+    .use(superscript_plugin)
 )
 md.add_render_rule("image", _image_rule)
 md.core.ruler.push("unwrap_lone_figures", _unwrap_lone_figures)

pagerite/themes/summer/theme.css

@@ -199,6 +199,12 @@ blockquote {
   padding-bottom: 0.25rem;
 }
 
+/* Admonitions join the rounded, softly shaded landscape elements. */
+.admonition {
+  border-radius: 0.6rem;
+  box-shadow: 0 0.2rem 0.7rem #4f913b1a;
+}
+
 /* Code rests on a shaded leaf panel, keeping the base light syntax
    palette. */
 pre {