Add documentation links and deployment effect

- Add API documentation links to all Cargo.toml files
- Add API documentation links to all crate READMEs
- Add API Reference link to main README
- Add Hercules CI effect to push docs to gh-pages on main branch

nix-bindings-expr-sys/Cargo.toml

@@ -6,6 +6,7 @@ build = "build.rs"
 license = "LGPL-2.1"
 description = "Low-level FFI bindings to the Nix expression evaluator"
 repository = "https://github.com/nixops4/nix-bindings-rust"
+documentation = "https://nixops4.github.io/nix-bindings-rust/development/nix_bindings_expr_sys/"
 readme = "README.md"
 
 [lib]