Add onboarding feature category for Growth:Activation

Why is this change being made?

To provide an appropriate feature_category for:

• First Mile (New user flow)
• Continuous onboarding

Related to gitlab-org/growth/team-tasks#338 (closed)

Checklist: https://about.gitlab.com/handbook/product/product-processes/#adding-a-new-category-to-a-stage

• add a ~Category:Onboarding label to gitlab-org - already exists https://gitlab.com/gitlab-org/gitlab/-/labels?utf8=%E2%9C%93&subscribed=&search=Category%3AOnboarding.

Review:

1. Categories: https://pcalder-growth-onboarding-feature-category.about.gitlab-review.app/handbook/product/categories/#growth-stage
2. Features (currently none for this category): https://pcalder-growth-onboarding-feature-category.about.gitlab-review.app/handbook/product/categories/features/#growthactivation-group

Note this adds the category for groupactivation but normal prioritization applies - i.e PMs/EM can discuss and move issues to another group.

@hilaqu as discussed, I need approval to add an onboarding feature category to the handbook.

cc @whaber @mkarampalas @s_awezec @jstava

Note this is not a significant change to categories or stages, so does not warrant the full approvals list https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/.gitlab/merge_request_templates/Category-Change.md - adding for notification purposes:

• Chief Product Officer @sfwgitlab
• VP of Product @adawar
• Director of Product Design @vkarnes
• Vice President of Development @clefelhocz1
• Director of Quality @meks => gitlab-org/quality/triage-ops#657 (closed)
• Vice President of User Experience @clenneville (cc @jackib)

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
• Assign this change to the correct DRI
  • 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 relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)