"Get involved" page content updates to reflect project progress

- Restored missing "Areas of collaboration" content
- Updated info about all parts of the project
- Clarified "our mission" section using bulle-points
- Updated "where we are" and "where we want to go next" in line with
  progress
- Distinguish between headings & links in community sidebar. dl elements
  need styling.

Closes #17
Moved some content to the about page; reorganised
Merge pull request #52 from openrightsgroup/get-involved-content-update

Content updates to reflect project progress