[UX] Add "Achievements" entry point to appropriate location in group's UI

Context

Achievements is going GA, but currently it requires users to know the url in order to create achievements and assign them to users. We need an entry point within a group which is visible for everyone.

• Only Maintainers and Owners can create, delete and award Achievements. Team is also implementing the ability to edit and revoke achievements.
• Users with other roles can only view group Achievements - no editing capabilities.
• All users, regardless of role, will be able to accept an achievement (work in progress, currently being proposed to accept through email invite for GA).
• All users will also be able to opt-out from displaying all achievements in their public profile - they won't be able to select which achievements will be visible, the setting will apply to all.

Requirement from UX

• Mock-up suggestions of appropriate entry point locations
  • Consider that, if considering adding to sidebar, Settings navigation is only visible to group Owners or Maintainers so the location needs to be role-agnostic
• If adding on sidebar, follow the handbook for the Requesting a navigation change process.
  • This could require rapid unmoderated usability testing focused on Owners and Maintainers creating and awarding an achievement.
• Attempt to avoid reliance on GitLab Docs.

Proposed entry points

Proposal	Owner/Maintainer view	Other roles view
Proposal A Entry point inside Group > Manage sidebar menu. Ideal for: Feature discovery and awareness Why here? The Manage section already houses role-agnostic items like Activity, Members, and Labels. Achievements fits naturally with these.
Proposal B Entry point inside Group landing page > Three-dots menu. Ideal for: Feature availability with minimal visual signal Why here? To be honest I wouldn't recommend adding here since the items here all include a verb for Actions (Copy, Edit, Archive, Delete), but it's an easy location with no impact to sidebar. This location will more than likely require awareness of the feature - either through releases or GitLab Docs.

Selected version Proposal A is included as a design in this issue