Improve documentation materials

[lyashevska]

@sjvrijn noted that when teaching materials they are still coming from different places, we need to revise them.

[lyashevska]

See also #29 and #44

[lyashevska]

Suggested layout:

1. Know what makes a good documentation (also mention fair)
2. Introduce Docstrings Add lesson material and an exercise for docstrings #46
3. README and CITATION.cff (mention cffininit) Add lesson material about CITATION.cff to documentation #47
4. Learn what tools (mkdocs, sphinx) can be used for writing documentation Add lesson materials and exercise for generating documentation with MkDocs #44 Consider adding a hands-on example on Sphinx #29

Resources:
https://coderefinery.github.io/documentation/sphinx/
https://carpentries-incubator.github.io/fair-research-software/09-code-documentation.html
https://book.the-turing-way.org/communication/citable/citable-cffinit.html
https://citation-file-format.github.io/cff-initializer-javascript
https://dev.to/cicirello/badges-tldr-for-your-repositorys-readme-3oo3
https://joshdimella.com/blog/python-docstring-formats-best-practices