Refactor Backend concept for data persistence

[s-heppner]

Warning

These changes are not backward compatible and therefore require a 2.0.0 release!

Previously, we tried to use the Backend concept for both data persistence and data integration.
This led to complicated cross-dependencies, architectures and badly maintainable code.

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 they are 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 Referable.update() and Referable.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]

For reference: Some of the discussions of this PR are at rwth-iat/basyx-python-sdk#58 as well.

[shenchenruwo]

Looks good to me