Refactor documentation and render on precice.org

[uekerman]

Let's create a page in the preCICE docs under Tooling named "FMI Runner". There, we want to render documentation (could be a single page), which is, however, hosted in this repo. Compare the OpenFOAM adapter or, very recently, the Micro Manager on how this works technically, i.e. what needs to be created on the website.

https://github.com/precice/precice.github.io/pull/270/files

In this repo here, we should the actual documentation page to a subfolder docs. The main README should then only contain minimal information ("what is this", "where to find out more") and link to the website. Also, content from other README pages could be ported to the docs.