Adding documentation on `.js` fixture files

[reinhrst]

(Taking a stab at) adding documentation on .js fixture files. Current documentation mentions that .js fixture files are possible, but doesn't describe how to use them, leading to issues like cypress-io/cypress#1271 .

I do think the current .js fixture support should get a good rethink (as mentioned 4 years ago in that issue) , but in the meantime it should have some documentation on how to use it (or fully remove any mention of it from the documentation, making it "unsupported").

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