docs: add documentation to the plugin plugin
docs: add documentation to the plugin plugin

These plugin docs will be good enough to tide end-users over until the 'plugin' plugin can be rewritten in golang.

Closes #4192

dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,dokku/dokku,progrium/dokku,dokku/dokku,progrium/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,progrium/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku