Production rollout of Organization sign-in
## Problem Statement
In https://gitlab.com/groups/gitlab-org/-/work_items/19425 we delivered the first iteration of Organization sign-in: a two-step flow for username/password.
We need to gradually roll out this feature on production for gitlab.com and communicate the change internally and externally as we move closer to active user adoption of Organizations.
## Exit criteria
* A user can sign in to an Organization they belong to by providing a username and password
* Documentation clarifies how sign-in works in an Organization context
* Developer documentation explains how this login flow works
## DRI
@peterhegman
epic