Remove docs about indexing

[cbkerr]

Description

Removing docs about indexing since it's deprecated.

Motivation and Context

See glotzerlab/signac#588

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.

[vyasr]

@atravitz @cbkerr any plans to move this forward?

[bdice]

@cbkerr @atravitz @vyasr I was looking at glotzerlab/signac#588 and realized that this PR solves most of the remaining work there. I think this PR looks good and will approve after a final review and checking the ReadTheDocs builds. I applied some minor changes but I don't think this needs further review since it's just a removal of the deprecated feature plus minor edits. In the interest of not stalling the work any further, I'm going to merge it soon -- we can revise anything else in later PRs if needed.

[bdice]

LGTM. Thanks for your work on this @cbkerr! I updated the signac-examples repo to remove indexing references. I'll merge this after verifying that ReadTheDocs builds look correct.