Doc/xr engine review cont.

[pawel-wolff]

This is a second (and hopefully final) part of the review of the docs concerning the Xarray engine:

• Docstrings for to_xarray() and open_dataset() revised: updates, formatting fixes, some improvements or clarifications added.
• Rendering a field as a HTML (_repr_html_()) fixed: now the notebook
  https://earthkit-data.readthedocs.io/en/release-1.0.0rc/how-tos/xr_engine/xarray_engine_to_grib.html
  is now rendered correctly on readthedocs.
• The remaining xr-engine how-to notebooks revised with a few minor fixes and improvements applied.

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• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.