typography and documents: formula and diagram SVG for export; a diagram label without a font is an error

src/documents/mod.rs

@@ -33,7 +33,7 @@ pub(crate) use blocks::source;
 pub use blocks::{Align, BlockData, BlockEditor, BlockEvent, BlockKind, Media};
 pub use contents::outline;
 pub use counts::{ReadingTime, WordCount};
-pub use diagram::Diagram;
+pub use diagram::{Diagram, diagram_svg};
 pub use document::{
     Attachment, DocumentEvent, DocumentHeading, DocumentLook, LARGEST_READ, MarkdownDocument,
     TableEdit, TableRefusal,