Add favicon and update navigation icons across documentation

- Added a favicon link to all tour pages in the Japanese documentation.
- Updated navigation links to include SVG icons for Home and GitHub.
- Changed language button to include an SVG icon for better visual representation.
- Improved theme toggle button to use SVG icons for light and dark modes.
- Refactored the documentation build commands in the justfile for clarity and consistency.

docs-gen/Cargo.toml

@@ -14,3 +14,9 @@ toml = "0.8"
 syntect = "5"
 anyhow = "1"
 clap = { version = "4", features = ["derive"] }
+notify = "7"
+tiny_http = "0.12"
+tungstenite = "0.24"
+open = "5"
+tempfile = "3"
+socket2 = { version = "0.5", features = ["all"] }