Update documentation on GitLab bots

Problem to solve

GitLab Admin Bot is an internal user that shows in customer instances and is documented here https://docs.gitlab.com/ee/development/internal_users.html

But there is nothing documenting:

• The features that the GitLab Admin Bot has access to and will be run.
• Details outlining that other users do not have access to access/edit the GitLab Admin Bot.

This gap in our documentation can impacts compliance focussed Ultimate customers specifically.

Proposal

Update our documentation with the features that GitLab Admin Bot actually does and clearly outline that the user can not be accessed or modified by others.

Investigation

Looking into the code base for this and found that the admin bot is being used at these places:

• Inactive project deletion - When an inactive project is deleted, the action generates an audit event that it was performed by the GitLab-Admin-Bot.
• Automatically deactivate dormant users - The action is audited that it was performed by GitLab-Admin-Bot
• Default compliance frameworks - The action is audited that it was performed by GitLab-Admin-Bot
• Locked users - By default, users are locked after 10 failed sign-in attempts. The action of locking the user is audited as it was performed by GitLab-Admin-Bot.
• Automatically delete unconfirmed users - The action is audited that it was performed by GitLab-Admin-Bot