Document differences with other measurement proposals

[johnivdel]

It would be good to maintain a comparison of ARA with other proposals in the measurement space (such as PCM, and IPA) to provide a high-level and digestible view of where the APIs are similar and different.

[dmdabbs]

PATCG may have produced something like that.

[csharrison]

We have https://github.com/patcg/docs-and-reports/blob/main/design-dimensions/README.md but it is really high level i.e. it does not discuss at all things like API surface details, etc it focuses more on "design dimensions" / architectural choices. Maybe that's good enough @johnivdel , but if you're interested in going deeper we should figure out if PATCG is a good venue for the document.