Reorganize documentation & update Readme

docs/Static-methods.md

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+# Meritoo Common Library
+Common and useful classes, methods, exceptions etc.
+
+# Static methods
+
+This package contains a lot of class with static methods, so usage is not so complicated. Just run the static method who would you like to use. Example:
+
+```php
+use Meritoo\Common\Utilities\Arrays;
+
+$firstElement = Arrays::getFirstElement(['lorem', 'ipsum']);
+var_dump($firstElement); // string(5) "lorem"
+```
+
+# More
+
+1. [Base test case (with common methods and data providers)](Base-test-case.md)
+2. [Collection of elements](Collection-of-elements.md)
+3. [**Static methods**](Static-methods.md)
+
+[‹ Back to `Readme`](../README.md)