Added missing cell execution docs for new schema

[MSeal]

A user noticed this wasn't updated in the read-the-docs. Doesn't need a release but will explain the schema changes and how to interpret them.

[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/nbformat-5-0-1-release/3001/3

[akhmerov]

execution probably also needs to be added to this table

[MSeal]

execution probably also needs to be added to this table

Which table are you referring to? I copied the pattern for the jupyter namespace docs above where the namespace is referenced in the description but not in the table. I could add a column with namespace so it's clearer when reading the tables?

[akhmerov]

Sorry, I expected the above comment would show as a line comment, but something didn't work. I mean this:

[MSeal]

I could add it there -- the table also doesn't have jupyter namespace specified either so I was being consistent to the docs as-is. It probably makes sense to add both jupyter and execution to that table.

[akhmerov]

Ah, so is it metadata.jupyter.execution, and not metadata.execution? I thought the latter...

[MSeal]

No it's metadata.execution. I was referencing that jupyter isn't in that table either and has the same pattern of nested values as execution. It has a dedicated section in the doc so I copied that pattern for the new namespace field. I'll just update the doc to make it clearer and see if it matches expectations.