text: Title: Document `column_default_sort` requires strings
  Body: ### Checklist
  
  - [X] The bug is reproducible against the latest release or `master`.
  - [X] There are no similar issues or pull requests to fix it yet.
  
  ### Describe the bug
  
  The examples in the [documentation](https://aminalaee.dev/sqladmin/configurations/#list-page) show `column_default_sort` as accepting class attributes. However, since https://github.com/aminalaee/sqladmin/commit/8f9d07d0705b241490a1f55fc69115ef598b8c50 `split()` is being called on them, and that fails unless they are strings
  
  ### Steps to reproduce the bug
  
  Visit an admin page with default sort defined via class attributes:
  
  ```python
  class UserAdmin(ModelView, model=User):
      ...
      column_default_sort = [(User.email, True), (User.name, False)]
  ```
  
  ### Expected behavior
  
  The default sort is applied as defined
  
  ### Actual behavior
  
  The server crashes because it can't `split()` class attributes
  
  ### Debugging material
  
  _No response_
  
  ### Environment
  
  Ubuntu 22.04, Python 3.11
  
  ### Additional context
  
  `column_default_sort = [("email", True), ("name", False)]` works as expected