Add documentation entry for "MyST vs Sphinx"

[agoose77]

In a The Turing Way working group meeting, it was put to me

What is the difference between MyST and Sphinx

I gave an answer, but I don't think it was concise enough. We should probably have something to hand as it's a natural question!

[jnywong]

Hey @agoose77,

I am sure you have given me a convincing explanation in the past, but I find the page under Reference > Background a useful reference! Maybe it's not very findable by users since it's buried beneath the Reference dropdown.