Can you find at least a project using that language on the web
That one is trivial. I can write webpages in esoteric programming languages, in fact, I've already done things like that.
Does the programming language have at least 10 page of documentation about it
Why would the quantity of documentation matter? Extremely simple languages barely need one page of documentation.
My feeling is that it's best to implement "soft" site-wide policies like not using built-in features of languages that automatically solve the bulk of the problem.
Why not? Golfing is about finding the shortest program in a language. If the language has a function which nearly does the job than that is the shortest way to do it.
The absolute shortest answer is secondary to me, and certainly a very short answer in a tool language that was designed for the domain of the problem isn't that interesting at all.
Again, if that is the case you are not golfing. It's not golfing if you just want some decent looking solution.
Are there two or more web-frameworks for serious use for the language?
That's no way to measure anything. Haskell has many good web-frameworks but is still considered (at least from the java/oop mainstream) to be a silly language nobody would use in production.
To sum up: If you want language agnostic golfing than some esoteric looking languages or even plain esoteric languages will win in some challenges. That's just the way it is. If you look at anarchy golf J kicks ass. It wasn't invented for golfing (probably ;)) but it turns out to be very good at it. Look at it as a sport. You won't set a maximum height for a basket ball player just because some are bigger than others and that turns out to be an advantage in some situations.
Otherwise create code golf problems and judge each language separately.
If you are not interested in golfing but interested in comparing your pseudo-leet-skills then just don't do golfing. Invent something else.