Heinrich Schuchardt authored Jan 16, 2020 and Tom Rini committed Jan 17, 2020

In reStructured text documentation we somtimes find formatting problems.
Therefore I would like to add 'make htmldocs' to our Gitlab Ci steps and
let all warning result in a build failure.

For building the htmldocs target without warnings we need the Graphviz
package.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>