[AETHER-76] Complete T3 offline mode

Regarding the T3 offline mode support, Troubleshooting Guide in the
trellis doc is updated (https://docs.trellisfabric.org/troubleshooting.html).
- Explanations on Network Information Base (NIB) and purposes of the offline mode support
- Usage instruction for the t3-load-file CLI command
- Usage instruction for the t3-load-snapshot CLI command

Change-Id: Iab8340ee50e539c40bf925ea1eec3a410357ff56
1 file changed