Add Best Practices source documentation in the google/transit repo

[isabelle-dr]

Context

MobilityData is in the process of consolidating the GTFS Schedule Best Practices and the GTFS Realtime Best Practices with the GTFS Schedule and Realtime specs that are part of this repo, because both serve as reference documentation that we aim to promote adherence to.

As part of this effort:

• all new Best Practices will be added directly into the spec.
• we are migrating some of the Best Practices based on their level of adoption and community consensus
  • phasing plan for migrating the GTFS Schedule Best Practices.
  • phasing plan for migrating the GTFS Realtime Best Practices.
• we've migrated issues and PRs that were still relevant from the Best Practice repos into this repo (list of issues here).
• we might work on restructuring the spec to smoothly incorporate Best Practices without adding complexity.

Included in this PR

This PR adds the Best Practice documentation into this repo to consolidate the reference documentation in this repo, and give the Best Practices more visibility.
I've also consolidated the GTFS Realtime Best Practices into one single document so it's easier to read.

When they are merged here, we will archive the Best Practice repositories.
We will leave this PR open for 10 working days, until May 9 end of day. If no opposition is expressed, we will merge it.

[Sergiodero]

Changes LGTM after a quick review! @isabelle-dr

[isabelle-dr]

Merging this PR, thanks for the review @Sergiodero!

As a result, we will:

• update the rendering of https://gtfs.org/schedule/best-practices/ and https://gtfs.org/realtime/best-practices/ to use the documentation from the google/transit repo instead of https://github.com/MobilityData/GTFS_Schedule_Best-Practices and https://github.com/MobilityData/GTFS_Realtime_Best-Practices
• archive the Best Practice GitHub repos (Schedule and Realtime)
• change links to the former Best Practice GitHub repos to refer to the corresponding files in the google/transit repo instead