doc: west: clean up example workspace

   zephyrproject/                 # west topdir

   ├── .west/                     # marks the location of the topdir

   │   └── config                 # per-workspace local configuration file

   ├── zephyr/                    # .git/ repo    │ the manifest repository,

   │   ├── west.yml               # manifest file │ never modified by west

   │   └── [... other files ...]                  │ after creation

   │

   │   # The manifest repository, never modified by west after creation:

   ├── zephyr/                    # .git/ repo

   │   ├── west.yml               # manifest file

   │   └── [... other files ...]

   │

   │   # Projects managed by west:

   ├── modules/

   │   └── lib/

   │       └── tinycbor/          # .git/ project