Clarify how to store extension attributes in the database

### Is there an existing issue for this?

- [X] I have searched the existing issues

### What's missing?

# General issue

Affected topic: /guides/v2.4/extension-dev-guide/extension_attributes/adding-attributes.html
Description:

The article is not clarifying whether we should create our own table column for scalar attributes manually for example via db_schema.xml or via an InstallSchema setup script to persist the extension attribute value in the database.
Possible solutions:

Clarify what a developer needs to do to store the extension attribute in the database.
Additional information:

Open /guides/v2.4/extension-dev-guide/extension_attributes/adding-attributes.html and review the existing extension-attribute guidance. Check how db_schema.xml and an InstallSchema setup script are described, then clarify the required way to persist scalar extension-attribute values in the database; done when a developer can determine whether to create a table column and how.

Written by the indexing model from the issue text.