docs: add "What are Configuration Classes?"

[kenjis]

Description
Closes #7388

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide

[datamweb]

After millions of days. 🌞

[kenjis]

This explanation is misleading because the Setting library does not affect the instantiation of Config objects.
I will reconsider.

[kenjis]

Settings can replace config() in application code, but not config() in the framework code.
So I removed the description.

It is perfectly fine to read and write the values of custom Config classes added (or added properties to existing config classes) by a dev with setting().

But even if we use setting() within an app to allow Cookie config to be stored in the database, it does not affect the code that retrieves the values as config(CookieConfig::class) in the framework code.

[kenjis]

If there are no objections, I will merge this within a couple of days.

[datamweb]

LGTM!