Update the extending/frameworks.md page with mkdocs-material

[PGijsbers]

We moved to mkdocs-material pages for documentation in PR #565, but I did not get around the extending/frameworks.md page yet. It would be great if the page could be updated to make it easier to read using mkdocs-material features such as admonitions and tabs. It also needs some careful reading to see if things are up to date, and perhaps there is room for improvement in general.

[PGijsbers]

Also add test coverage for the example, like adding a script which follows the step in an automated way to ensure that the docs are up-to-date. To avoid a situation like #593.