Add BackendEntrypoint to an easier to find location

[alexamici]

Is your feature request related to a problem? Please describe.

I started implementing a few backend plugins following the documentation so I noticed a few easily fixable quirks:

http://xarray.pydata.org/en/latest/internals/how-to-add-new-backend.html

The main one is the BackendEntrypoint is found deep into xarray.backends.common.

Describe the solution you'd like

I suggest to add an easier to find reference:

• xarray.backends.BackendEntrypoint would do
• but maybe even xarray.BackendEntrypoint.

Additional context

For the simplest plugins BackendEntrypoint would be the only import to connect to xarray.

See this dummy GDAL plugin: https://github.com/bopen/xarray-gdal/blob/main/xarray_gdal/xarray_plugin.py
But also the rioxarray plugin in this PR: corteva/rioxarray/pull/281

[alexamici]

Same with BackendArray for plugin that want to support lazy loading.

[shoyer]

👍 for putting all external API stuff related to backends in the top level xarray.backends.

[alexamici]

This didn't close automatically, but it is closed by #5127