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>

src/documents/mod.rs

@@ -1,5 +1,6 @@
 mod blocks;
 mod counts;
+mod diagram;
 mod epub;
 mod find;
 mod format;
@@ -9,6 +10,7 @@ mod link;
 mod markdown;
 mod modes;
 mod pages;
+mod picture;
 mod print;
 mod reading;
 mod render;
@@ -28,6 +30,7 @@ use gpui::App;
 pub(crate) use blocks::source;
 pub use blocks::{Align, BlockData, BlockEditor, BlockEvent, BlockKind, Media};
 pub use counts::{ReadingTime, WordCount};
+pub use diagram::Diagram;
 pub use epub::{Chapter, EpubReader};
 pub use format::{Format, format};
 pub(crate) use history::Change;