text: Title: Update documentation to match changes with v8.x.x
  Body: ## Describe the bug
  
  Some of the documentation has mismatched method names. For example, the documentation under the `Usage -> Routes` page still references an `after_register` handler, but that is now the `on_after_register` handler within a `UserManager`.
  
  ## To Reproduce
  Go to the [Usage > Routes Page](https://fastapi-users.github.io/fastapi-users/usage/routes/#post-register) to see some of the mismatched method names.
  
  ## Fixes
   - I opened #768 to address this.
   - The examples at [https://fastapi-users.github.io/fastapi-users/configuration/full-example/](https://fastapi-users.github.io/fastapi-users/configuration/full-example/) also have invalid `UserManager` classes and method names, but those examples are not in this repo for me to fix.
  