Update docs/source/index.rst

I've reworked the introduction some changing the introduction structure to

- Tell them what this is ("this doc [is about] writing and executing acceptance tests")
- Give them reassurance there are in the right place ("acceptance test[s] ... are ... tests which check out the user interface or through the web testing")
- Give them a little more reassurance and a little more information without trying to confuse them ("... by using Robot Framework and Selenium. Robot Framework is ... .Selenium is ...")
- Lend them right into an example (note the rearrangement of the contents with robotsuite.rst on top) while giving a link to those who wish to work core 

I know this is probably the wrong place for THE introduction to Plone's acceptance testing but for now this seemed like the place where we were making THE Introduction so I reworded it some. -Ed