Navigation and anchor links formatting for GTFS Realtime reference

[emmambd]

Feature request

Is your feature request related to a problem? Please describe.
GTFS Realtime documentation heavily relies on the element index, which has a nested structure. Right now the navigation for GTFS Realtime reference is not nested, which makes things confusing, especially when one submessage is duplicated and nested in multiple messages (e.g ScheduleRelationship).

Proposed solution

• Update the Markdown and yaml nav config so the navigation and links are nested and clearly highlight the appropriate relationships, e.g https://gtfs.org/realtime/reference/#TripUpdates.msg-TripDescriptor.enum-schedulerelationship.

• Stretch goal: update the navigation so it's nested

Describe alternatives you've considered
It doesn't look like anything fancier is needed aside from some info architecture changes, but this may be more complex to solve after more exploration.