Add getting started development docs using included examples

Add getting started development docs using included examples. Some useful tips I used while developing (thanks @jaime) !336 (merged)

There are some existing docs for developing GitLab pages inside the GDK: https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/pages.md

TODO

• After merge: Add a section similar to geo development in the main nav which would link to https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/pages.md
  • gitlab-development-kit!1545 (merged)
  • gitlab-docs!1156 (merged)