Add Bumble documentation

[JPHutchins]

Update docs about transports. Explain how to get HCI FW as an extra.

[ethan-mark-osu]

I'd like to take this one. To scope it: should the Bumble/transport docs go in the module docstrings, a new mkdocs page, or the README transports section?

[JPHutchins]

Thank you! Most of the docs are in the source code itself and then the mkdocs generates docs from that. Explaining use of the hci_firmware extra would be nice, ideally flashing a nrf52840dk with nrfutil. Pairing with a PIN requires user interaction (or a programmatic OOB read, e.g. the program reads the PIN over serial).

I've not tested this transport on "ready made" dongles that bumble supposedly supports.

[JPHutchins]

There's a little app built in - smpbumble - that can scan, pair, etc. more integration work is in progress over at smpmgr.