Update contributing with more info about images

[svx]

Explain that we do certain images with robot-framework and such stuff.

[stevepiercy]

References: https://github.com/plone/documentation/issues/822#issuecomment-295943326 and #984

We have something for manually adding images in the Documentation Styleguide, but I couldn't find anything about the automatic generation of screenshots via robots. How does a new contributor know when to add an image manually versus letting the robot take a screenshot automatically?

[stevepiercy]

Closing as won't fix. We currently do not automatically generate screenshots in Plone 6 docs. We might add it in the future, but there are no plans to do so.