Changes

We shouldn't always build our documentation afresh.
Indeed, if you run `./cleanall` and `./make` then OpenCOR will build
fine. However, if you rerun `./make` straight after that, you will be
told that `[OpenCOR]/ext/doc/user` (or `[OpenCOR]/ext/doc/developer`,
depending on which one gets built first) doesn't appear to contain a
CMakeLists.txt file.
So, instead, we have to update our `./clean` script to always clean our
`[OpenCOR]/ext/doc` folder.