Added Validation/Build workflow summary table from Plan issue

Why is this change being made?

Through various conversations across Eng, Design, and Product within Dev/Plan we discovered that many individuals had different understandings of each development phase and who the DRI for each should be. We set out to define, discuss, and agree on this in an effort to drive clarity and unity during the Product Development process here at GitLab.

Plan, in https://gitlab.com/gitlab-org/plan/issues/34, created a nice summary table of the Product Development Flow that's worth codifying in the handbook. 👍

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

When this gets merged...

• Update workflow::start to workflow::triage at GitLab.org group level.

Approvals

Per the change approval process:

• VP of Product @sfwgitlab
• UX Director @clenneville

FYI

• Senior Director of Development @clefelhocz1
• Director of Quality @meks