Add CODEOWNERS file

What does this MR do?

Add a CODEOWNERS file. The only entries are the docs team for now.

Related issues

Discussed in gitlab-org/quality/triage-serverless#124 (comment 607719675). This is a first step to have the bot ping the docs team for community contributions.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• [-] MR has a green pipeline on GitLab.com
• [-] Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• [-] trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• [-] Test plan indicating conditions for success has been posted and passes
• [-] Documentation created/updated
• [-] Tests added
• [-] Integration tests added to GitLab QA
• [-] Equivalent MR/issue for the GitLab Chart opened