Various fix about documentation

Context

After the call with Alexia and Valentin, here are points to improve on the documentation :
Related to their issue : hub#503

• Precise how to use a never image with a job. They speak about a new variable field, I don't think it's a good idea. Precise how we can specify a new image on the job, according to the documentation
• Remove the dropdown section far too long and text too small on the MR :
  
• Specify more precisely how resolve threads and when asks for a review.

Objective

Modify the Contribute page of the doc to mentions those points.