Use CommonMark instead of Markdown in sample notebooks

Problem

Julia's current markdown library has numerous unresolved issues and the heavy use of the library in Pluto.jl is creating numerous follow on issues in the Pluto.jl project for what is an integral, but unrelated dependency:

Proposed Solution

Fortunately, there is a new, nicely architected (and well-tested) library, CommonMark.jl which can be used as a (nearly) drop-in replacement in the sample notebooks (ignoring for the moment the particular use cases for Pluto.jl internals. This library is under consideration for inclusion in Julia's stdlib and forms an easier to reason about basis for making notebook computing 'simpler and more accessible'.

Note

As far as I can tell, the CommonMark is missing a macro to make strings 'just work' as markdown text. If this proposal is accepted, this macro could potentially be added to the Pluto.jl project and then eventually integrated into the CommonMark.jl package.

I have converted one notebook in a PR as an example so that you can assess the overall impact of this change.

https://github.com/fonsp/Pluto.jl/pull/956

fonsp / Pluto.jl