doc: move initialise docs higher up nav menu/toctree (#488)

Previous order:
- How-to Guides
  - Install MicroCloud
  - Update and upgrade
  - Manage the snaps
  - Initialise MicroCloud

New order:
- How-to Guides
  - Install MicroCloud
  - Initialise MicroCloud
  - Update and upgrade
  - Manage the snaps

doc/how-to/index.md

@@ -7,9 +7,9 @@ These how-to guides cover key operations and processes in MicroCloud.
 :maxdepth: 1
 
 Install MicroCloud </how-to/install>
+Initialise MicroCloud </how-to/initialise>
 Update and upgrade </how-to/update_upgrade>
 Manage the snaps </how-to/snaps>
-Initialise MicroCloud </how-to/initialise>
 Configure Ceph networking </how-to/ceph_networking>
 Configure OVN underlay </how-to/ovn_underlay>
 Add a machine </how-to/add_machine>