Fix documentation problems

The three commits fix the following three problems respectively:

  * Change the generated documentation directory to `docs/html`, `docs/latex`, etc.
  * Use the [qmi style][1] for documentation instead of the default.
  * Add `docs/description.md` and `docs/overview.md` to the generated documentation and set `docs/description.md` as the default page.

  [1]: https://github.com/skozlovf/doxygen-qmi-style