Add adapter configuration strings & restructure adapter method docs

[calpt]

Configuration strings

This PR adds the possibility to use flexible adapter configuration strings which allow specifying custom config attributes.
Examples:

• Set config attributes: model.add_adapter("name", config="parallel[reduction_factor=2]")
• Config union model.add_adapter("name", config="prefix_tuning|parallel")
• more examples: https://github.com/calpt/adapter-transformers/blob/8df62b9de2a8ab51115b191aca35b2fb53c96539/tests_adapters/test_adapter_config.py#L95-L102

Documentation: https://github.com/calpt/adapter-transformers/blob/8df62b9de2a8ab51115b191aca35b2fb53c96539/adapter_docs/overview.md

Configuration strings can allow passing complex configurations e.g. via command line.

Documentation restructuring

The adapter method documentation is now split into three pages:

• Overview and Configuration: introduction, table, configuration
• Adapter Methods
• Method Combinations

[hSterz]

Looks good to me