Add documentation on how to use ZBT-1 as OTBR with container installation

[seppeel]

The problem

At first thanks for the great documentation page for the connect zbt-1!
I want to buy and use the connect-zbt1 as open thread border router but i am not using the home assistant OS - i am using the container installation type.

the documentation does not contain any information on how to flash the thread firmware and start the OTBR with containers.

Im pretty sure that i will manage to do this somehow by using the silabs firmware builder and building a container image with otbr-posix but i would like to benefit from the efforts you already put into the HA addon (automatic firmware upgrade and i see some patches?)

so i would appreciate if you could write some documentation on how to use the existing ressources from https://github.com/home-assistant/addons/tree/master/openthread_border_router

Link to the page

https://connectzbt1.home-assistant.io/procedures/enable-thread/