Add user docs for Pages access control

What does this MR do?

GitLab Pages access control user docs. We already had admin docs, so I also added user ones.

• Write new section user docs in pages/introduction.md, link to admin and vice versa.
• Update permissions.md to link to the user docs instead of the admin ones.
• Mention that this is not enabled on .com.
• Update image in https://docs.gitlab.com/ee/user/project/settings/index.html#sharing-and-permissions

When this will be enabled on .com, we will have to remove the CORE ONLY badge.

Related issues

• Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/33422
• Admin docs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22146
• Permissions docs change https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22798

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54143

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards