Update handbook for Matt Manela and team structure

[mmanela]

Changes

• Add myself to people list
• Update pages for Product Platform group

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[sourcegraph-bot]

Notifying subscribers in CODENOTIFY files for diff 7667cf2...8860468.

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[ErikaRS]

Thanks for the updates! I commented on a couple of things that can wait until a separate PR if you prefer. Either way, no need to wait for another round of review.