📐 Add check to avoid 'undefined' in generated documentation

[ccamel]

Purpose

The documentation we produce (thanks to fadroma/schema - #311) relies on a specific format of comments. For instance, if types aren't correctly described in the comments of the source code, they'll appear as undefined in the documentation.

Todo

To maintain documentation quality, we should integrate a check process that ensures no undefined definitions are present in the committed documentation.

There are several ways of doing this, but it could be an additional control step in the makefile when generating the documentation (which will also be used in the CI).