Create CODEOWNERS file to begin formalizing docs responsibilities

[felker]

@BethanyL and @FilippoSimini are leading a reorganization of the effort for creating and maintaining (at least the Data Science group's) documentation. First step is to identify and record (in some centralized place) the points of contact for all pages and topics.

Suggestion: add a CODEOWNERS file to either .github/ or docs/, which can serve as that centralized place instead of some Confluence/Box location; plus it will automate requests for PR reviews when the identified files are modified by commits in the PR.

Example: https://github.com/PrincetonUniversity/athena/blob/master/.github/CODEOWNERS