Better intro for new users in docs

[johnomotani]

Some thoughts from @mikekryjak that we should follow up on:

I think the examples are great, but they assume that the user already has some experience in xarray.
I would have the following recommendations for the documentation:

• Start with even just one simple paragraph that explains what xarray is and what its benefits are (e.g. super intuitive metadata and indexing)
• Have a small section to explain basics of indexing, or include a link to the relevant xarray doc page (which is actually very well written)
• Have a bullet point list of all the plotting tools that xBOUT adds with a single sentence description
• Have a bullet point list of all the native xarray plotting tools with a single sentence description

If anyone has other suggestions, please add them!