Add in the contents of the wiki to `/doc/`

- On good advice, I'm committing the contents of the current wiki into code, so that the history of our docs can be reviewed in git, and so we have an offlne source.
Change links to point at the `/doc/` folder rather than wiki
Add a ToC for the `/doc/`

- Just a basic list of what's inside the folder for quick reference
Merge pull request #118 from alphagov/feature/add-documentation

Add current wiki documentation to `/doc/`