Add lesson materials and exercise for generating documentation with MkDocs

[ClaireDons]

Add an (optional) exercise for generating documentation. Could be an example where participants can choose between sphinx, mkdocs and roxygen.

Exercise where you add docstrings to temperature conversion code. (This would require a working example with no docstrings). This could then be used in the CI example.

[lyashevska]

https://carpentries-incubator.github.io/fair-research-software/09-code-documentation.html