Make capitalization of section headings consistent

[bdice]

We're inconsistent about capitalization in section headings across the entirety of the signac documentation. We use title case in some places and sentence case in other places. I think I would prefer to use sentence case instead of title case.

Examples:

• "Selecting jobs for aggregation" is sentence case.
• "Selecting Jobs for Aggregation" is title case.

Originally posted by @bdice in #122 (comment)