Update Documentation for backend Implementation

[aurghs]

The backend read-support refactor is drawing to a close and we should start to add the documentation to explain how to implement new backends.

We should:

• decide where to put the documentation
• decide a title
• define a brief list of the main points to discuss in the documentation.

For the first point, I suggest putting the documentation in "Internal".
For the second one, I suggest: "How to add a new backend"

Concerning the third point, in the following there is a list of the topics, that I suggest::

• BackendEntrypoint Description (BackendEntrypoint is the main interface with xarray, it's a container of functions to be implemented and attributes: guess_can_open, open_dataset, open_dataset_parameters, [guess_can_write], [dataset_writer])
• How to add the backend as an external entrypoint.
• Description of the function contained in BackendEntrypoint to be implemented. In particular, for open_dataset we have two option to describe:
  • No Lazy it returns a dataset containing numpy arrays.
  • Lazy it returns a dataset containing BackendArrays:
• BackendArrays description:
  • thread-safe __getitem__
  • Pickable (use CachingFileManager)
  • indexing.IndexingSupport

[alexamici]

Closed by #4810