doc: fix "Getting Started" structure for section docker

Fixes two problems:
- Section "Usage" which describes the usage of docker is on heading level 1 but has to be on  heading level 2
- Section "Setup" has only one subsection. A second subsection was added. Now, the section "Setup" has two subsections, one that describes the installation of docker and one that describes downloading and testing of the RIOT docker build image.

doc/doxygen/src/getting-started.md

@@ -308,6 +308,8 @@ The user on your computer requires permission to access and use docker. There ar
 - Your OS distribution may create a group called `docker`. If so, then adding yourself to that group (and logging out and in again) should grant you permission.
 - Execute docker with sudo. This is in fact the most secure and recommended setup (see [here](https://docs.docker.com/install/linux/linux-postinstall/), [here](https://docs.docker.com/engine/security/security/#docker-daemon-attack-surface), [here](https://www.projectatomic.io/blog/2015/08/why-we-dont-let-non-root-users-run-docker-in-centos-fedora-or-rhel/) and [here](https://fosterelli.co/privilege-escalation-via-docker.html)). No extra setup steps are needed. `make` should be instructed to use `sudo` by setting `DOCKER="sudo docker"` in the command line.
 
+### Downloading and testing RIOT docker container
+
 Finally, download the pre-built RIOT Docker container:
 
 ```console
@@ -321,7 +323,8 @@ This will take a while. If it finishes correctly, you can then use the toolchain
 $ docker run --rm -i -t -u $UID -v $(pwd):/data/riotbuild riot/riotbuild ./dist/tools/compile_test/compile_test.py
 ```
 
-# Usage
+Usage
+-----
 
 The RIOT build system provides support for using the Docker container to build RIOT projects, so you do not need to type the long docker command line every time: