Update _index.md

[cphills33]

1. We refer to the questionnaire player by many names. This could confuse readers (e.g., minno-quest, miQuest, PIquest, the player). Perhaps all these terms refer to different things, but I can’t find a page that describes what each of these is and how they differ. I haven’t changed these because I’m not sure what the best to resolve this is.
2. The ‘eventually’ link doesn’t work. It goes to a ‘Not Found’ page. I have updated the link.
3. I rephrased the central concepts section to more clearly convey the steps needed to create a questionnaire.