add doctest examples for core xarray APIs

[nicpittman]

This PR Implements part of the request in issue #6793 by adding simple and runnable doctest-ready examples for a subset of core APIs. Other methods noted in the issue remain out-of-scope for this PR. It also fixes a couple of outdated/broken examples and adds some more See Also links.

New examples were added for some of my favourite methods including:

• load_dataset
• load_dataarray
• open_dataset
• open_dataarray
• open_groups
• open_mfdataset
• infer_freq
• polyval
• decode_cf
• Dataset.compute
• Dataset.persist
• Dataset.drop_dims
• get_options
• unify_chunks

This PR fixes several outdated or inconsistent doctests: updating the monthly frequency example in save_mfdataset and modernising See Also references.

Happy to adjust formatting (e.g., skip flags, repr looseness, stylistic preferences) based on maintainer feedback.

• [x] References #6793
• [x] Tests added (doctests across multiple modules)

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