Updated SUMMARY.md and fixed hyperlinks

Hi,

Firstly apologies for the size of this pull request. It contains widespread changes to hook up all the existing raw content into a workable Gitbook. I've fixed all (?) the hyperlinks between pages according to the directory structure you established.

I also rebuilt the SUMMARY.md based on the existing ThoughtWorks help documentation and ensured all pages are included in the Gitbook build.

Cheers, Andy