Merge "[docs] initial Thread Border Router vendor guide" into main am: e1b7abd0c4 am: 290f4e3dfb

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

Change-Id: I04761b73f96bd12b4e7af55baeb0ba78c64256f7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>