Clarify pydata#8728 in docs

[ks905383]

• Add reference to numpy docs on view / copies in the corresponding section of the xarray docs, to help clarify pydata#8728 .
• Add note that other xarray operations also return views rather than copies in the Copies vs. Views section of the docs
• Add note that da.values() returns a view in the header for da.values().

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[ks905383]

I think the failing check is something that came from the latest main branch update, so once that gets resolved, I'll merge this barring any other comments?

[ks905383]

does anything else need to happen before it's cleared for merging btw?

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