Add a 'lesson' object

[svenvanderburg]

We currently add lessons that follow The Carpentries format (meaning they are open-source version controlled web based lesson materials) on the research software directory as 'software'. See an example here: https://research-software-directory.org/software/introduction-to-deep-learning-lesson

It would be great if there would be a separate 'lesson' object. Which would allow:

Filtering just on lessons
Referring to the object as 'lesson' instead of 'software': Currently it for example says: 'Cite this software' this should become 'Cite this lesson'
In terms of other features it can be similar to the 'software' object

[BSchilperoort]

Related to this, it would be nice to have a "lesson" or "lesson material" category in project output/mentions.