docs: Add styling for "Deprecated" admonitions

Style the `deprecated` admonition class like a warning, but with the
`material/grave-stone` icon and its own colour, so deprecation notices
read as deprecation notices and not as generic warnings.

That class is what a hand-written `Deprecated:` admonition in
a docstring produces, and also what the griffe extension added next
will emit, so a single rule covers both sources.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

docs/_css/mkdocstrings.css

@@ -42,3 +42,25 @@ a.autorefs-external::after {
 a.autorefs-external:hover::after {
   background-color: var(--md-accent-fg-color);
 }
+
+/* A "Deprecated" admonition, styled like a warning but with its own icon. */
+:root {
+  --md-admonition-icon--deprecated: url('data:image/svg+xml;charset=utf-8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M10 2h4c3.31 0 5 2.69 5 6v10.66C16.88 17.63 15.07 17 12 17s-4.88.63-7 1.66V8c0-3.31 1.69-6 5-6M8 8v1.5h8V8zm1 4v1.5h6V12zM3 22v-.69c2.66-1.69 10.23-5.47 18-.06V22z"/></svg>');
+}
+
+.md-typeset .admonition.deprecated,
+.md-typeset details.deprecated {
+  border-color: #cc9900;
+}
+
+.md-typeset .deprecated > .admonition-title,
+.md-typeset .deprecated > summary {
+  background-color: #cc99001a;
+}
+
+.md-typeset .deprecated > .admonition-title::before,
+.md-typeset .deprecated > summary::before {
+  background-color: #cc9900;
+  -webkit-mask-image: var(--md-admonition-icon--deprecated);
+  mask-image: var(--md-admonition-icon--deprecated);
+}