Delete outdated base.md file

docs/internals/base.md

-Base classes and interfaces
-===========================
-
-Object
-------
-
-Object is the base class for many other Yii2 classes.
-
-### property feature
-
-#### Why
-
-To be able to make property `public` initially and then seamlessly make it
-`private` or `protected` by adding getter and setter method. That will *not*
-change API. Results in less repetitive code. Performance drop isn't significant.
-
-### callbacks and expressions
-
-### [[Object::newInstance|newInstance]] method
-
-This method is a powerful way to instantiate a class. Differences from `new`:
-
-- Calls class constructor (same the `new` operator);
-- Initializes the object properties using the name-value pairs given as the
-  last parameter to this method;
-- Calls [[Initable::init|init]] if the class implements [[Initable]].
-
-#### Why
-
-To support class dependencies and their lazy loading.
-
-### [[Initable]] interface
-
-Developer will implement initable interface if running `init()` needed and will
-skip it if not.
-
-#### Why
-
-Indicates where `init()` will be called and where not. More explicit than it was
-in Yii 1.
-
-Component
----------
-