Update Achievements docs

Problem to solve

We have a UI where you can view and create group achievements (https://gitlab.com/groups/gitlab-org/achievements/-/achievements) that is not documented. That page is not present in the menu, it can be accessed only directly at the URL.

Proposal

In Achievements docs:

• Add a section View group achievements
• In the section Create an achievement, document the flow from the UI.

Who can address the issue

TW for grouptenant scale

Other links/references

Slack discussion