Clean up "Contributing to LoopBack" page

[dhmlau]

Description

The current Contributing to LoopBack page only contains information on how to contribute in LB3. We should include info on how to contribute on LB4 as well.

Acceptance Critera

Clean up the contributing pages for LB3 and LB4 because they are different:

• create "Contributing to LB3" page, possibly with the current content
• create "Contributing to LB4" page, possibly https://github.com/strongloop/loopback-next/blob/master/docs/CONTRIBUTING.md
• current "Contributing" page to point to the above 2 pages

[dhmlau]

Closing as done.
Contributing code instruction and contributing docs instruction have been updated.