clarify specification of CD URIs and CDURL for linked data

[kohlhase]

see OpenMath/OM3#127

[davidcarlisle]

This issue is discussing the different CD formats that could be made at the same canonical URL distinguished by mime type. Given that the current plan is to host the canonical URLs at github pages which doesn't offer this feature then we need to park this issue (or look to a different website host).

[kohlhase]

You are right, I will clear the milestone which parks it to the future.