Add integration test and helper method documentation

Description

This MR adds documentation for creating integration tests, as well as descriptions of the currently available integration tests and helper methods.

I added a document for the helper methods. While the descriptions are similar to the JavaDoc (so there is probably a more fitting way to handle it), this document is intended to make assumptions and page loads of the methods more clear at a glance.

The documents are added in a new "docs" folder, as Markdown files.

Closes

Closes #174 (closed)