Backend - Documentation: Project structure

**Describe the solution you'd like**
The file structure and important files of the backend-template should be documented and described.
A table-based documentation which short information about the purpose of folders and files must be created.

**Describe alternatives you've considered**
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**Additional context**
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Inspect the backend-template tree first, then identify its folders and important files and their purposes. Create table-based documentation covering that structure, and verify that the table reflects the current template contents.

Written by the indexing model from the issue text.