Updates to Dask page in Xarray docs

[scharlottej13]

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

Hi! It's been a while since the Dask docs page had a comprehensive update, so thought I'd open a PR with some suggestions.

I chatted a little bit about updating this page with @jhamman and @dcherian, some of the things I was trying to keep in mind:

• Include more examples
• Organize information around what the user is trying to do
• Update outdated info

There are certainly a lot of things I wasn't sure about (I'm certainly not an xarray expert). I'll comment in-line with questions.

cc @phofl @jrbourbeau

[welcome[bot]]

Thank you for opening this pull request! It may take us a few days to respond here, so thank you for being patient. If you have questions, some answers may be found in our contributing guidelines.

[scharlottej13]

@shoyer @TomNicholas thank you both for the review!

I believe I've addressed all of your comments, but please let me know if I missed anything.

[scharlottej13]

@shoyer @TomNicholas gentle ping here, thanks again for the thorough review!

[scharlottej13]

@shoyer @TomNicholas another gentle ping here, thanks again!

[dcherian]

Sorry for the really long delay here @scharlottej13

[welcome[bot]]

Congratulations on completing your first pull request! Welcome to Xarray! We are proud of you, and hope to see you again!

[scharlottej13]

Thank you @dcherian!!