What context installation do you use? The reference is the one on the context garden. Each year tex lives has a snapshot of that one. So, first make sure you run the latest version.

Concerning old manuals: they often refer to mkii but with mkiv we have different (often better) solutions. The examplap code and examples are mkii and also relate to pdf trickery and as pdf evolved it became more clear what was bound to acrobat i.e. not picked up (ignored) by open source alternatives and therefore less relevant.

An important source are the setup-*.pdf files as these describe the interface which is described in the interface definition files (in xml format). These have recently be updated by Wolfgang and are very accurate.

You mentioned that keys (functionality) that disappears should be somehow documented but normally no functionality disappears. What happened was that mkiv has some more (because it's possible) and less (because it was no longer needed) than mkii and has been made more consistent.

The wiki has a page for examples and you’re free to update the
existing examples or add new ones.

Do you mean http://wiki.contextgarden.net/Sample_documents ? Yes, that's
a good idea, since I believe that it is a way to improve documentation
with little efforts. Then I plan to add the cover letter style I made
since I am happy with the results, although 1 or 2 things could have
some improvement. There are also some letter styles there, then I will
have a look on them too. I don't know if people are intimidated about
writing contents in the wiki because it is the official wiki and feel
that only programmers are supposed to update it, but it will be really
helpfull if the users have more the habit of posting there the document
styles they made.

Indeed.

Sounds interesting and the best thing you can do is to start with it.

Thanks for your encouragement ! Yes that looks like an interesting
challenge for me, but it is not something I am wanting to do alone
because of my lack of experience, at least I would need someone to coach
me. Actually I don't have really experience with web developpment and I
would at least need help for the technological choices. Having someone
that tells me that if I use technology X, there are module Y and Z that
will be a good fit is a good start.

This will only work when someone takes the lead. If you do that you then others will help you. If you need something special on the wiki, just discuss it with Taco and Mojca who deal with the technicalities.