Problems with documentation (outdated stuff + dead links)

Multiple TODOs:

• Outdated: https://frihsb.gitlab.io/rettij/docs/working-with-rettij.html#device -> "The type defines the class of the node." -> replace type by device
• Multiple links in the rendered readthedocs documentation are 404, e.g. at the main page: https://frihsb.gitlab.io/rettij/index.html -> the "GOTO" links are dead because they link to the .md version of the docs, not the .html version. However, simply replacing all the links by html doesn't solve the problem, because the docs would stop working in the repository browsing view where you'd browse through the source markdown files. I guess there must be some kind of solution that automatically replaces the links with correct file extensions