We're going to build the entire website together to simplify maintenance and
cross-linking. There is no need to maintain each "book" (building-with-duim,
getting-started-ide, ...) as separately buildable documentation projects.

At a high level, this renames directories as follows:
```
documentation/website/source/* -> documentation/source/*
documentation/*/source         -> documentation/source/*
```
while dropping all but the top-level Makefile and conf.py.

We use Furo now.