Fork Sonarr plugin into its own package

[Callum027]

• Remove base Buildarr source code, and move Sonarr plugin source code into a new buildarr_sonarr module
• Change package metadata to be for the Sonarr plugin (including adding a dependency for Buildarr core)
• Refactor documentation so that only the Sonarr docs are available
• Update Poetry to 1.4.1
• Write release notes for the Buildarr Sonarr plugin, and update old release note links
• Temporarily disable Mypy pre-commit hook as it is not consistent with running it locally (to investigate in another issue)