Add new pages for mirroring options

In gitlab!70992 (merged) https://docs.gitlab.com/ee/user/project/repository/repository_mirroring.html is moving to https://docs.gitlab.com/ee/user/project/repository/mirror/ and three new child pages are being created:

• https://docs.gitlab.com/ee/user/project/repository/mirror/bidirectional.html

• https://docs.gitlab.com/ee/user/project/repository/mirror/pull.html

• https://docs.gitlab.com/ee/user/project/repository/mirror/push.html

• Link to the merge request of the new page: gitlab!70992 (merged)

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page. gitlab!70992 (merged)