Removal of the Dokkatoo plugin, integration with the `:clean` task

### Dokkatoo plugin

The Dokkatoo plugin has been removed (#19, !37). Please migrate to the Dokka v2 plugin instead.

### Dokka plugin

- The `:clean` task now deletes all files generated by the plugin (#20, !38)
- The plugin automatically generates a `.gitignore` file in the `/api` folder (!38)