Merge "[docs] initial Thread Border Router vendor guide" into main am: e1b7abd0c4

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3189671

Change-Id: I241b8dca3cf6d634e1172f779599d4b79c40dfaf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>