Produce how to videos for building an example in Espresso.

[jwhhh]

More:

• [x] for the validation tests, document more clearly on the criteria and to write better error message
• [x] use a table to describe the build and validation steps (description, tools used, source code)

Useful reference: https://documentation.divio.com/

[jwhhh]

Video draft: https://www.dropbox.com/s/j1xdl0q7zj039cl/%28with%20clicks%29%20how-to-contribute-an-example-in-espresso.mp4?dl=0