Add back mcumgr documentation as unsupported option

[beriberikix]

https://golioth-docs-dev--pr187-szczys-update-view-l-epck9zjn.web.app/hardware/nrf91/quickstart/flash-firmware/bootloader

I'm creating this as a placeholder while we're planning on the updating doc plan wrt verification levels.

While we aren't currently including mcumgr/newtmgr as part of our new levels of verification, it is still supported by Nordic in NCS and more generally by many boards in Zephyr.

We should reintroduce the instructions but indicate that it is unsupported at this time.