Improve documentation for `SLURMRunner`

[moi90]

The repo where SLURMRunner was originally implemented contains valuable information (especially the "Runner vs Cluster" section) that should be included in the dask-jobqueue documentation. #638 and dask/community#346 also contain information that might be valuable to a user.

[guillaumeeb]

Hi @moi90, thanks for raising this!

@jacobtomlinson already put up some summary in the landing documentation page (https://jobqueue.dask.org/en/latest/), but I guess improvements could be made, or adding specific sections!

Do not hesitate to propose some improvements to the doc!

[moi90]

I would say it wouldn't hurt to repeat and extend the information that is already on the landing page. (If I get to the documentation via search a search engine, it is unlikely that I read it front to back first, but rather I expect to find all information on the topic on the same page.)

[jacobtomlinson]

Sounds totall reasonable to link back to that @moi90. Do you have some time to put in a Pull Request with the improvements you are proposing?