Markdown: display math through RaTeX, Mermaid through mermaid-rs-renderer, pictures from the owner's folder

Latex typesets with RaTeX to outlined SVG drawn by gpui's SvgRenderer at the
window's scale, loaded as an asset off the main thread. Diagram draws a
mermaid block in Mermaid's theme for the mode. MarkdownRenderer::base names
the folder pictures load from; data addresses decode, web ones stay unloaded.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

examples/gallery/pages/typography/values.rs

@@ -204,7 +204,7 @@ pub fn latex(cx: &App) -> impl IntoElement + use<> {
     let math = |source: &str| Latex::parse(source).expect("gallery formula parses");
     section(
         "Latex / MathInline",
-        "A TeX subset: scripts, fractions, roots, Greek.",
+        "TeX typeset by RaTeX, in the text's color.",
         cx,
     )
     .child(