Update HighLevelGraph documentation

I just came across this page of the docs which hasn’t been changed much since Dec 2018. I think it would be useful to update it to include a runnable example and some information about the Layer class.

Once that page is updated, it will probably make sense to update https://docs.dask.org/en/latest/phases-of-computation.html and https://docs.dask.org/en/latest/graphs.html to make it clearer how HLGs fit in.

I am happy to take on this work, but just wanted to put this here in case I am missing something.

Some resources that I am planning to draw on: Layer Issue Blog post about scheduling Scheduling workshop from summit

Pinging the people who I have seen working on HLG related things: @rjzamora @GenevieveBuckley @gjoseph92 @ian-r-rose @jrbourbeau

[ian-r-rose]

For what it’s worth, I also recently wrote up some notes on HLG pain points. It’s less detailed than Rick’s but perhaps still interesting

[rjzamora]

@martindurant @douglasdavis - The following gist is still very rough for now, but may be helpful for to-be layer developers: https://gist.github.com/rjzamora/3f76c6dcbde16a4185222eaadcd1d02f