updating get involved section on how gov works and linking to get involved page

Agreed changes with Neil. Change from list to 3 columns and alter copy.
https://www.pivotaltracker.com/story/show/44946303
Merge pull request #414 from alphagov/get-involved-update

updating get involved section on how gov works and lined to get involved

https://www.pivotaltracker.com/story/show/44946303