Adds improvements for Pages ACL docs

What does this MR do?

Adds 2 minor changes to GitLab Pages ACL docs:

• the section Access control links to https://docs.gitlab.com/ee/user/project/pages/introduction.html#gitlab-pages-access-control-core but it will be more correct to link user to https://docs.gitlab.com/ee/user/project/pages/pages_access_control.html
• the section GitLab Pages Access Control mentions Settings > General > Permissions as a path while the correct one is Settings > General > Visibility, project features, permissions.

cc @marcia