Improve/data persistence

[shenchenruwo]

Previously, we tried to use the Backend concept for both data persistence and data integration.
Now, we decided to split these two concepts and use the existing Backend ObjectStores for just the data persistence.

Concretely, this PR does the following:

1. Remove the abstract Backend class (backends.py), as well as their concrete implementations (CouchDBBackend, LocalFileBackend), as it is no longer needed
2. Remove the Referable.source attribute, as it was only needed for the Backend class
3. Remove methods related to the Referable.source attribute, such as update() and commit() (base.py), as they were used together to perform the data integration.
4. Adapt other related codes such as tutorials and tests to reflect these changes
5. Adapt documentation to reflect these changes

[s-heppner]

See eclipse-basyx/basyx-python-sdk#370.