Moving Realtime Best Practices into the Spec: Phasing Plan

# Problem

This issue mirrors issue #396 created for merging the Schedule Best Practices into the spec, but the scope here is GTFS Realtime.
MobilityData’s heard several pains from the community about the GTFS best practices and the spec’s SHOULD statements living in two different places:

- Producers do not always refer to the best practices, and so moving these into the official spec would give the best practices greater visibility and improve data quality for everyone
- Merging the best practices in the spec would make it easier for regulators to point producers to one place to get the information they need to create their GTFS feeds

Based on this feedback, MobilityData is working on merging the best practices into the official spec, and to point new best practice discussions to https://github.com/google/transit.

# Phasing Plan
MobilityData has created a phasing plan to merge the Realtime Best Practices into the spec.
Each phase will include a corresponding PR to remove parts of the Best Practice documentation.
Any community members are welcome to open a PR to merge best practices — the goal of this phasing plan is for everyone to be roughly aligned on steps so anyone can move to action.

Phase # | Best Practice section | What’s included
-- | -- | --
1 | Feed Publishing & General Practices. | Adding them as a new section in the spec, mirroring the similar section in the GTFS Schedule spec
2 | Practice recommendations organized by message. | Add recommendations to either the file or the field descriptions.
3 | Practice recommendations organized by use-case (frequency-based trips) | Add to the corresponding spec locations:

• TripUpdate/StopTimeUpdate/StopTimeEvent
• TripUpdate/TripDescriptor
• VehiclePosition/TripDescriptor

  # Additional context and related issues
  - #396
  - #375
  - #376