docs: add @immutable to Config classes that do not extend BaseConfig

[kenjis]

Description
There is no reason to change the values after the instantiation.

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

[neznaika0]

https://phpstan.org/r/d471fb72-a06c-4798-9d19-9e34901b7458
Readonly properties are allowed in the parent constructor.
phpstan shows errors as:

 ------ ------------------------------------------------- 
  Line   Config/Autoload.php                              
 ------ ------------------------------------------------- 
  :42    @readonly property cannot have a default value.  
         🪪  property.readOnlyByPhpDocDefaultValue