Expand/collapse functionality for sub-headings in the left nav

[eric-schneider]

Headings in the left nav have expand/collapse functionality and default to collapsed state on page load. Sub-headings should behave the same way. However, currently, you can't expand/collapse sub-headings at all, and they default to open.

For example, see the sub-headings in the Databases section:

[colegoldsmith]

Closed by #69