The SDK documents are updated each release cycle.
Are you reading the Photoshop CS6 SDK, or some much older SDK version?
No third party groups that I know of. The SDK and the example source are the normal documentation.
Yes, I have looked at the CS6 SDK. The problem is the documentation on Automation Plugin development, including recent SDK's still include documentation dating back 13 years (or more) regarding the subject. The Automation Tutorial PDF is from the Photoshop 6.0 and references development tools no longer in use such as CodeWarrior. The Actions Event Guide PDF dates to Photoshop 5.5 in 1999.
You'd think current information referencing modern tools would be available.
the best way to learn is to look into sample code + use Listener and Getter
Some of the old documents are really god for start
(naturally path of files or file names mentioned are not ok but if you use text search trough files in sdk you will be fine)
Scripting primer.pdf - how to make plugin "actionable"
Automation Tutorial.pdf - name says it
Photoshop Actions Guide.pdf - also good for start learning (understanding) but to write code in later SDK se HTML documentation
Photoshop API Guide.pdf - is also good for understanding process of making plugin but to make code look into HTML docs
PICA.PDF - About suites , what are they acquiring releasing etc (similar to COM interfaces or DirectX ...)
Adobe really should update those but or just make clear that they are just for learning not coding.