WIP: Initial draft of quad planning docs

Why is this change being made?

Adding documentation to help define the quad planning process.

Closes https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/481 Closes https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/125

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

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