Add documentation and UI text labels/considerations to Product Development flow

Adds the following steps to the Build track > Build phase 1: Plan:

• Include documentation requirements (per Development docs and Feature proposal template).
• Add the new documentation label (recently renamed from ~Documentation) (per Development docs).
• Add the new ~"UI text" label.

Note: The ~"UI text" label is helpful per recent additions to the UX handbook:

• During Product Design, if UI text may be considered, UX designers are encouraged to consult the Technical Writer.
• During reviews for changes that include UI text, UX designers involve the Technical Writer.

This will help address gitlab-org&1679 and ensure greater efficiency and quality for documentation work.