I have a working SSMS extension here: https://github.com/ErikEJ/SqlCeToolbox
In particular: https://github.com/ErikEJ/SqlCeToolbox/tree/master/src/GUI/SSMSToolbox/SSMSEngine
For versions before 18, you also need to have your extension id on a list of approved extensions, so it is unlikely that you will be able to do that.
The reason why there is no documentation is that is is basically an unsupported (by technically possible) scenario.