Don't build docs for the bin target

The library contains documentation covering both CLI and library usage,
there's no separate documentation for just the binary target.

Cargo.toml

@@ -20,6 +20,7 @@ path = "src/lib.rs"
 [[bin]]
 name = "obsidian-export"
 path = "src/main.rs"
+doc = false
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html