Formalizing notion of self-directed time to handbook for Principal+ ICs

Why is this change being made?

We want our members of our IC branch of our Engineering dual career track to be happy and engaged, and believe autonomy is key to that. There is also a company need to allow them to focus on some of our hardest problems for which they are the best DRIs.

In order to iterate, this experiment has been scoped to Principal+ (15 people), but if successful we may bring it to Staff as well (an additional 54+ people) in a subsequent iteration.

CC @stanhu @ayufan @grzesiek @glopezfernandez @andrewn @plafoucriere @clefelhocz1 @JohnathanHunt @sloyd @marin @timzallmann @whaber @cdu1 @tstadelhofer

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---