Check code examples in entity descriptions as part of CI

[byorgey]

Is your feature request related to a problem? Please describe.
When making updates for #303 , we almost missed the fact that we needed to update some code examples in entity descriptions (@noahyor caught it). When updating the entity descriptions, I caught several other things that needed to be updated which had been out of date for a while.

Describe the solution you'd like
Code samples in entity descriptions ought to be automatically checked in our CI process (by parsing and typechecking them), just like code in the examples/ directory is checked.

I am not quite sure how this would work. There would need to be some special way to mark code examples in entity descriptions to make it possible to extract them during the CI process. Then we would have to process them specially when displaying entity descriptions. Maybe it's as simple as inventing some special delimiters, then stripping those delimiters when displaying entity descriptions.

[byorgey]

Another benefit of specially delimiting code examples in entity descriptions would be that we could syntax highlight them (#574).

The simplest idea might just be to use triple backticks like Github-flavored Markdown. In fact, we might consider using Markdown in general.