Fix markdown files according to markdownlint recommendations

There are some issues and quirks in the markdown documentation files
suggested by the markdownlint project checker that might benefit from
being fixed, which this patch does.

Change-Id: I33245825e5bb543b5ce1732204984d4a0b169668
Signed-off-by: Joakim Roubert <joakimr@axis.com>