Add minimal glossary

[cbkerr]

Description

My goal is to define the terms for signac-core "good enough" to get started to keep this moving.

I think the new hover extension is worth it to allow people to see the definition when hovering.

Motivation and Context

#59

Work continues at https://docs.google.com/document/d/1_merhcK3ohas4IloE616yL7gypMRFcaQLh2oChExC7o

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My code follows the code style guideline of this project.

[b-butler]

Is this meant to cover all uses of the glossary terms?

[b-butler]

@cbkerr you need to ensure that the readthedocs environment has the hoverxref plugin.

[cbkerr]

Is this meant to cover all uses of the glossary terms?

Yes, these terms shouldn't have multiple definitions.

I tried to pick the ones that had most settled in the google doc.

[b-butler]

I more meant, that many of the terms do not use hoverxref in the rest of the documentation.