Add initial guide on monorepos

[nojaf]

Fixes #1043

[vercel]

@nojaf is attempting to deploy a commit to the ReScript Association Team on Vercel.

A member of the Team first needs to authorize it.

[fhammerschmidt]

I guess we can and should ditch this page: https://rescript-lang.org/docs/manual/v12.0.0/build-pinned-dependencies
But its contents might still be handy, e.g. for explaining the build rules here?

[fhammerschmidt]

@jfrolich anything to add from you POV?

[fhammerschmidt]

@nojaf now that you are a rewatch expert. ;-)
Anything to add here?

[nojaf]

@fhammerschmidt yes, this is mostly inaccurate I'm afraid.
Things work differently than past me anticipated.

[fhammerschmidt]

Absolute banger!