Automatically test the getting started instructions

[amirmc]

We should be able to run tests on the instructions for building mir-www (and any others). I'm thinking of something similar to the 99 problems page on OCaml.org (source), but in reverse. i.e. the code snippets on the page are pulled in from version-controlled source files elsewhere (via some templating magic).

xref: #299, #267, #238

[yomimono]

There are folks currently working on this for the new edition of Real World OCaml. I hope we can repurpose their work here (and elsewhere in the Mirage ecosystem).

[samoht]

I've added the necessary machinery to do this in #817 using mdx. The rest of the work is now tracked by #770 so closing.