Create a glossary of terms for Parsl documentation

[WardLT]

Is your feature request related to a problem? Please describe.
The many workflow systems vary in terminology they use, and Parsl is not special of this. We should create a Glossary for terms and make it prominent in the documentation for key concepts, such as:

• Block
• App
• Job
• Worker
• Executor

Describe the solution you'd like
A documentation page with one sentence explanations for each term.

Describe alternatives you've considered
Linking people to the Quickstart, where I've tried to explain each term. However, the narrative form of that document is not great for quick reference.

Additional context
This would be great for our Outreachy team member :D

[benclifford]

cc @sophie-bui

[WardLT]

Once we have the glossary, we should ensure consistency. I think we're good on using the same words, but I'm worried

[benclifford]

other concepts i've found poorly separated:

• task
• (worker) pool
• manager (of workers)
• app invocation - vs task, vs (definition of an) app

[benclifford]

• node

... is a great one because it has two completely different meanings: node in the graph-theory sense of a piece of a DAG, if you have DAG-brain engaged; and something similar to what Internet Protocol people would call a host that executes code. for example, disambiguate this: a node is executed on a node.

• resource

this gets loosely used in the same way that people use the word asset elsewhere - often to mean not much more than "thing".