It is required to add Moodle docs for the plugin. There need to be docs for all branches 2.3 - 2.9.
Possible approaches:

1. Create an initial version of the plugin documentation that is applicable for all versions and upload a copy of it to each docs version. Continue to add version specific features or behavior as needed.
2. Maintain one version of the plugin documentation and highlight differences between the plugin versions in that documentation. The documentation pages from all other versions will always redirect to the latest version.