Where is the official documentation?
The source code for several clockfaces and apps is available on github.
The official documentation contains a link to a generous set of assets (icons, fonts, etc).
React (.jsx) files are used to manage your clockface/app settings on the companion. Here's a good general explanation.
Here's the official Fitbit SDK forum.
There's an unofficial Discord chat board here.
Both places are frequented by Fitbit staff and third-party developers.
If you want to be ultra-helpful, try to provide the following information:
If your suggestion is for a new feature, you can post in the Feature Suggestions forum.
The official list of issues with the SDK is here.
You can contribute to the SDK by following the link on that page.
Fibit OS 1.2 - 184.108.40.206 (Ionic)
Fibit OS 2.0 - 220.127.116.11 (Ionic) or 18.104.22.168 (Versa)
Fitbit OS 2.1 - 22.214.171.124 (Ionic) or 126.96.36.199 (Versa)
Fitbit OS 2.1.1 - 188.8.131.52 (Ionic)
Fitbit OS2.2 - 184.108.40.206 (Ionic) or 220.127.116.11 (Versa)
Fitbit OS 3.0 - 18.104.22.168 (Ionic) or 22.214.171.124 (Versa)
The official Fitbit SDK development roadmap is here. Unfortunately, it isn't kept totally up-to-date.