docs: Make emscripten path warning more obvious.

[Gadgetoid]

This bit me when I was testing #792 - hopefully splitting the configure and build steps, and decorating the step with a ⚠️ will make it more obvious.

[Daft-Freak]

Looks more obvious to me.

(I wonder if enough time has passed that we can recommend cmake -B build instead of the mkdir/cd dance 🤔. That's probably for another PR though.)

[Daft-Freak]

"CMake" here I think

[ahnlak]

I know the Emscripten docs tell you to use emmake make rather than a naked make, although functionally it ends up the same (I assume?)

Also, isn't it more 'Emscripten-y' to use emrun instead of doing the python web thingie by hand?

[Daft-Freak]

Not using emmake usually works if you don't depend on environment variables at build time. (CMake's makefiles hardcode pretty much everything).

Using emrun would probably be a good idea, at least so stuff doesn't break if security gets more strict again (it sets all the magic headers so that things like threads work). Does have the disadvantage that you have to specify an example though.