[web] Document the need for yarn classic

[mnvr]

People often run into issues with yarn install because they're using a newer yarn. The situation is generally bad - we don't want to update to Yarn v4 yet because it is marked experimental and is not the default yarn that gets installed by node currently. We could add a packageManager field to our package.json, but this will only fail the build with a better (hopefully) error message, and will necessitate the user to corepack_enable.

I'm not sure what's the best approach right now to make the initial setup be seamless (I think we're using the approach that works for the maximum of all the alternatives, but I'm not sure). At least, let me add a note about it.

Ref:

• [Feature] Add a warning or error in Yarn Classic if package.json's packageManager key is set, but corepack is not enabled yarnpkg/berry#5912