Docs improvements (#7660)

* Docs: update sidebar

* Docs: move registry documentation into docs/

* Docs: move rbd_provisioner documentation into docs/

* Docs: move cephfs_provisioner into docs/

* Docs: move local_volume_provisioner documentation into docs/

* Docs: move ambassador.md to docs/ingress_controller/

* Docs: move metallb.md to docs/ingress_controller/

* Docs: move ingress_nginx documentation into docs/

* Docs: move alb_ingress_controller documentation into docs/

* Docs: merge ambassador documentation into docs/ingress_controller/

* Docs: move cert_manager documentation into docs/

* Docs: move bootstrap-os documentation into docs/

* Docs: update file locations in sidebar