Add link to the wiki

- Adding this quite high up as readers may just wish to get to the implementation details immediately, having seen the intro speil before.
Quick reweite of the language para

- very minor clarification of language, probably just subjective here!
Change the language from 'cover sheet' to 'start page'

- We have Start Pages on GOV.UK and I think everyone is familiar with this terminology
- Not sure if there's a separate language within content teams wherein 'Start Page' gets translated into 'Cover Sheet', but I think 'Start Page' works for a broader range of people within a team.
- Adds liknks to wiki entries for start pages and questions. Placeholders for the other types
Move 'cover sheet', 'question' and 'choice' to the wiki

- They're at https://github.com/alphagov/smartdown/wiki/Start-Pages, https://github.com/alphagov/smartdown/wiki/Questions and https://github.com/alphagov/smartdown/wiki/Choices now
- Have also been rewritten to include examples of smartdown, html and rendered content