This commit includes many updates to our comparison chart with
other resource managers:
 - update the Features list to include some things we don't
   support but other resource managers do, such as official
   packages,
 - update to include license information for other RMs,
 - include footnotes and links to feature tracking and relevant
   definitions,
 - reword some of the features to tighten/clarify what they mean,
 - and finally, drop the comparison to explicit workflow
   managers, given that Flux isn't one and isn't trying to be.