docbuild CI: check commit message for sth like [nodoc]

[tfiers]

It takes 30 minutes to build the docs.

Say 50 watts for the build VM? (that's like a small laptop)

So 25 watt-hour.
An oven is 2 kWh.

So 80 docbuild runs are like leaving an oven on for one hour

GH says it's carbon neutral (but doesn't specify how; could be fishy)

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this issue proposes a system where, if you remember, you can add a magic phrase like [nodoc] somewhere at the bottom of a commit message, to not have the docbuild run (A lot of commits -- even those outside test/ -- do not merrit a new docbuild)