Update CODEOWNERS for Product Intelligence transition

What does this MR do and why?

Update the CODEOWNERS file to make @fneill the primary reviewer for Snowplow and Service Ping documentation.

Related issue: technical-writing#478

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.