Where is the official documentation?
Here.
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).
Most of the 'smarts' in your clockface/app will be written in JavaScript. Here's a tutorial about it, and here's a good general reference. Beware that the JavaScript implementation used by Fitbit is customised and is not based on the latest standards, so Fitbit's official documentation is the final word.
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.
Alas, no. Search the forums and Discord (as best you can). If no joy, ask in one of those places.
In lieu of a publicly-accessible bug tracker, your best bet is to post on the forum or Discord. Fitbit staff see posts in either place, but may not acknowledge or respond.
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.
SDK 1
Fibit OS 1.2 - 27.31.1.29 (Ionic)
Fibit OS 2.0 - 27.32.4.19 (Ionic) or 32.32.4.19 (Versa)
Fitbit OS 2.1 - 27.32.10.15 (Ionic) or 32.32.10.15 (Versa)
Fitbit OS 2.1.1 - 27.32.10.20 (Ionic)
SDK 2
Fitbit OS2.2 - 27.32.12.19 (Ionic) or 32.32.12.19 (Versa)
SDK 3
Fitbit OS 3.0 - 27.33.1.30 (Ionic) or 32.33.1.30 (Versa)
The official Fitbit SDK development roadmap is here. Unfortunately, it isn't kept totally up-to-date.