Link API docs to user guide and other examples

Noting down a comment by @danjonesocean on Twitter: https://twitter.com/DanJonesOcean/status/1441392596362874882

In general, having more examples on each xarray page (like the one below) would be good. Then they would come up quickly in function searches:

http://xarray.pydata.org/en/stable/generated/xarray.Dataset.merge.html#xarray.Dataset.merge

Our API docs are generated by the function docstrings, and these are usually the first thing users hit when they search for functions. However, these docstring uniformly lack examples, often leaving users stuck.

I see two ways to mitigate this:

• Add examples directly to the docstings (suggested by @jklymak)
• Cross reference other examples from the user guide or other tutorials

Start by reviewing the generated API documentation and the user guide or tutorials referenced in the issue, especially the Dataset.merge example. Determine which API docstrings lack examples and whether existing guide examples can be cross-referenced. Done should mean the agreed set of API pages provides useful links or examples, with the generated documentation checked for correctness.