Add Mermaid diagram support with SVG export

- Add mermaid code block detection and rendering
- Integrate Mermaid.js v10 via CDN for diagram rendering
- Add SVG export with download button on hover
- Add preview command to open markdown in browser
- Add documentation for all supported diagram types

Cargo.toml

@@ -7,7 +7,8 @@ edition = "2021"
 pulldown-cmark = { version = "0.10", features = ["html"] }
 clap = { version = "4.5", features = ["derive"] }
 anyhow = "1.0"
-syntect = "5"
+dirs = "5"
+open = "5"
 
 [[bin]]
 name = "md_reader"