chore(dev): Clarify CODEOWNERS requirements and add additional owners (#16404)

This updates CONTRIBUTING.md to mention that CODEOWNER reviews are required but also adds secondary
reviewers for each code owner that is an individual to cover the case where the primary reviewer is
inaccessible (e.g. on PTO).

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>