Create new teams for Dev Backend <=> Product mapping

In order to better align with our Product categories, we're reorganizing our Discussion and Platform teams into four new teams:

Instead of managing Discussion, @smcgivern will now manage Plan, and will manage Share on an interim basis until we hire a new manager for that team.

Instead of managing Platform, @DouweM will now manage Create, and will manage Admin on an interim basis until we hire a new manager for that team.

@oswaldo and @fjsanpedro are switching managers in this move so they can continue to be subject matter experts for these new areas, and I appreciate their flexibility. Everyone else has been assigned new teams per the update in the merge request.

I've also updated our hiring plan (on the team page) and KRs accordingly, and updated the team pages in the engineering section of the handbook (although there's not a lot there yet).

While we want to go ahead and communicate this change, we appreciate that this kind of reorganization takes a little bit of transition time, so @DouweM and @smcgivern are working through these changes with their teams and we expect them to be fully transitioned by the start of the 11.3 release cycle.

/cc @edjdev and platform/discussion devs: @felipe_artur @jarka @jprovaznik @mdelaossa @crollison @lulalala @rdavila @jameslopez @tiagonbotelho @jamedjo @reprazent @ifarkas