Revise first pages, combine concepts and motivation

[rkdarst]

• Combine concepts and motivation pages, so that the first parts
  actually can be done quickly.
• A lot of the content has been combined into a list in a few sections
  (e.g. all former sections about types of tests is now one section
  with a smaller list)
• Discussions slightly adjusted, but hopefully similar in spirit.
• These sections are no longer a testing guide - it should get to
  examples quickly. I still have the old pages and they could be
  re-inserted somewhere if desired, like optional stuff at the end?
• Review: worth a quick read to make sure someone agrees that the
  important stuff is there.