Mark new community-contributed features in the docs

Originated as this Slack thread (internal to GitLab)

Details

For community contributions, a few times I've done this:

> [Introduced]() as a community contribution in [GitLab Starter]() 13.3.

I think it highlights that people are contributing to GitLab, and might make the contributor happier and maybe encourage someone in the future. Should we add it to the style guide?

Summary of discussion / points of view until now

• It's a better idea to use this for whole features which would require a version note anyway, and not for every community-contributed improvement.
• A mention in the release post is enough.
• If it's a big enough feature to have its own section with an "Introduced in" note, it makes sense to say who added it. It is a very very small pat on the back to someone that may have spent dozens of hours on a feature, who then gave it to us for free, and lets everyone else use it.
• It can promote development of more fully fledged features from the public if we point out features added by contributors.
• It could help any conversations regarding issues with the feature, or future changes to it, if it's clearer that it was added by a contributor.

Approaches

• Mention in the version note that a feature was added as a community contribution.
• As above, but also call out and link the contributor's profile. We can easily make it optional by adding a suggestion into the MR:

  Would you mind if we add your name next to the feature?
  Click apply on the first suggestion.
  If you'd like your name out of the docs, click apply on the second suggestion.