Fix table of contents in docs site

[thekaveman]

Let's align the Eligibility Server docs site table of contents to how it is presented in the Benefits docs.

Acceptance criteria

• We have a single consolidated section-level menu that appears on the left side of the docs site
• An auto-generated table of contents for each page appears inline with the section-level menu for a given page

Additional context

See cal-itp/benefits#889 for how this was done in Benefits