ddh_intro.md: market to interested users.

Believing this page to be targetted to encouraging people to contribute:

- Explain how instant answers have benefited them as users.
- Make it clear that everyone, regardless of skill level, can provide
  valuable feedback.
- Show that the feedback actually leads to a better experience for everyone.
- Ask them to help.
- Create a consistent hierarchy of changes (better sources -> more
  answers -> coding on answers)


This is a much more controversial pull-request than I have made previously.  It tries to focus the page on my opinion of how the page is targeted, imposes my opinions of what motivates people and, perhaps worst of all exposes open@duckduckgo.com to more naive scrapping.

I looked this over many many time, but I suspect that a spelling error/typo still exists.