The new version requires `python3` to be available. In omnibus
installation this is already a reality as we are currently pathing
previous `gitlab-markup` version to use `python3` instead of `python2`.
We are now requiring `python3` with the gem without having to patch it.
As a consequence to also make it easy to use it in development, we've
introduced a `Pipfile` and `Pipfile.lock`, working similarly to
`Gemfile` and `Gemfile.lock`, and added documentation on how to use
them.
This removes duplicated content from CONTRIBUTING.md and consolidate
everything under doc/development/contributing/.
This also fixes missing links.
Lastly, this moves the style guides from the "Implement design & UI
elements" page to a new "Style guides" page.
Signed-off-by: Rémy Coutable <remy@rymai.me>
