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.

meson.build

@@ -1,6 +1,6 @@
 project(
   'depot',
-  version: '0.18.0',
+  version: '0.18.1',
   meson_version: '>=0.60.0',
   default_options: ['buildtype=release'],
 )