Update docs for devise install

[cenotaph]

Following the gitbook docs, for a Rails 7.0.2.4 API-only app with devise_token_auth 1.2.1 and devise 4.9.0, without running rails generate devise:install before rails g devise_token_auth:install User auth, then attempting to migrate the db or do anything will result in the error:

NoMethodError: undefined method `devise' for User:Class

Whatever devise:install does, it seems to be needed so it might be good to add that atop the 'Configuration' page on the gitbook docs.

Thanks for making and maintaining a great gem!

[Samuelodan]

Sadly, this hasn't been implemented till now. Thanks for pointing it out.