Contribution guidelines need update

[osrf-migration]

Original report (archived issue) by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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• There are outdated links in the style section pointing to default when they should be pinned to a commit to make sure the line doesn't change
• Some info is outdated, like the dashboard
• While most of the tutorial applies to Gazebo and the Ignition libraries, some parts of it don't, like a big chunk of the "Internal Developers" section. It may be worth it pointing that out, because we do use this document as a reference for Ignition a lot of times.