Adds docs for the theme labeling pilot

References: Product#3907

Why is this change being made?

• This is part of the Customer Issues Prioritization Framework Pilot and is a necessary process in order to aggregate epics/issues at the Top ARR driver and Product Investment Themes levels.
• This process will also be in a limited pilot in conjunction with https://about.gitlab.com/handbook/product/product-processes/customer-issues-prioritization-framework/#pilot-context

Proposed Label Additions

Note: We are intentionally keeping the scoped label keys generic and not specific to any given year. This keeps the key evergreen and saves us from having to constantly update the underlying data model as themes and investments change over time.

Themes:

• theme::sass availability
• theme::horse
• theme::security testing
• theme::compliance events
• theme::fedramp
• theme::sod
• theme::code review
• theme::open shift
• theme::geo improvements
• theme::sass performance

Investments:

• investment::hosted first
• investment::workflow usability
• investment::ci/cd leader

Next Steps

Once this MR is merged and the initial set of labels are :thumbs-up: :

• Open an issue with engineering productivity to automate the mapping and application of the investment:: label based on the corresponding theme:: label.
• Open an issue with the data team to add aggregation by theme and investment to the prioritization model and relevant dashboards.
• Update this documentation to reflect and include the above two action items.

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.

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