Bump version to 0.18.1 and implement documentation splitting feature

- Updated version in Cargo.lock and Cargo.toml to 0.18.1.
- Added functions to handle documentation package creation and management in `src/package/spec.rs`.
- Implemented logic to split documentation into separate outputs during the staging process in `src/staging/mod.rs`.
- Enhanced database interaction to retrieve package directories in `src/db/mod.rs`.
- Refactored command handling in `src/commands.rs` to accommodate new output specifications.
- Added tests to verify the correct behavior of documentation splitting and package management.

Cargo.lock

@@ -418,7 +418,7 @@ checksum = "807800ff3288b621186fe0a8f3392c4652068257302709c24efd918c3dffcdc2"
 
 [[package]]
 name = "depot"
-version = "0.18.0"
+version = "0.18.1"
 dependencies = [
  "anyhow",
  "ar",