Move from ReadTheDocs to Mkdocs by leveraging Github Pages

[mlodic]

Mkdocs reference: https://github.com/mkdocs/mkdocs

We could host the site via Github Pages. See how OpenCTI does it here: https://docs.opencti.io/. They use this style

In this way we would have a centralized place for documentation and we can change it without the need to make a release in the Project itself.

All the documentation regarding the list of plugins name can be kept inside this repo to avoid to have the contributors that create a new analyzer to update 2 repos at once.