Document how an IoT compose becomes a milestone

[coremodule]

We currently don't have documentation that shows how we pick which IoT compose will become a milestone (beta, final). We do have a process that we follow, but it hasn't been written down.

Link to the IoT Weekly Meeting where this was brought up:
(https://meetbot.fedoraproject.org/fedora-meeting/2024-01-10/fedora_iot_working_group_meeting.2024-01-10-16.00.log.html)

[nullr0ute]

Assigning to you @coremodule as we discussed in the meeting you documenting it from the meeting discussions.