UtUtils class -- doc added #930 (#1189)

* UtUtils class -- doc added #930 * Minor fixes in content and formatting

Author: olganaumenko

docs/UtUtilsClass.md

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+# UtUtils class
+
+## What are the utility methods
+
+_Utility methods_ implement common, often re-used operations which are helpful for accomplishing tasks in many 
+classes. In UnitTestBot _utility methods_ include those related to creating instances, checking deep 
+equals, working with arrays, using lambdas and so on — miscellaneous methods necessary for generated tests.
+
+## Why to create UtUtils class
+
+Previously UnitTestBot generated _utility methods_ for each test class when they were needed — and only those which were necessary for the given class. They were declared right in the generated test class, occupying space. Generating multiple test classes often resulted in duplicating _utility methods_ and consuming even more space.
+
+For now UnitTestBot provides a special `UtUtils` class containing all _utility methods_ if at least one test class needs some of them. This class is generated once and the specific methods are imported from it if necessary. No need for _utility methods_ — no `UtUtils` class is generated.
+
+## What does it look like
+
+Here is an example of a comment inherent to every `UtUtils` class:
+
+<img width="494" alt="ututils" src="https://user-images.githubusercontent.com/64418523/196719780-2603f141-e922-40fc-9a0a-533aaacc5c49.png">
+
+As one can see, the comment mentions two characteristics of the `UtUtils` class:
+
+1. _Version_
+
+If the generated tests require additional _utility methods_, the
+existing `UtUtils` class is upgraded and gets a new version number, which should be defined here:
+
+`org.utbot.framework.codegen.model.constructor.builtin.UtilClassFileMethodProvider.UTIL_CLASS_VERSION`
+
+_2. Mockito support_
+
+UnitTestBot uses [Mockito framework](https://site.mockito.org/) to implement mocking. When generated tests imply mocking, the 
+`deepEquals()` 
+_utility method_ should be configured — it should have a check: whether the compared object is a mock or not. That is why the `UtUtils` class for the tests with mocking differs from the one without mocking support.
+
+If you have previously generated tests with mocking, the next `UtUtils` class will support mocking as well — even if 
+it is upgraded or current tests do not need mocking, so that the existing tests can still 
+rely on the proper methods from `UtUtils` class.
+
+## Where to find it
+
+`UtUtils` class is usually located in the chosen **Test sources root** near the generated test classes.
+
+## How to test
+
+If you want to test `UtUtils` class generation using UnitTestBot project itself, refer to the [Manual testing of 
+UtUtils class generation #1233](https://github.com/UnitTestBot/UTBotJava/issues/1233).
+
+## How to improve
+
+UnitTestBot does not currently support generating tests for classes from multiple modules simultaneously. If this option was possible, we would probably have to generate a separate `UtUtils` class for each module. Perhaps we could find a special location for a `UtUtils` class reachable from every module.
+
+For now, you can generate separate `UtUtils` classes for different modules only if you manually choose the different **Test sources roots** when generating tests.