Introduce a beta channel

[szaimen]

Useful for after the stable release

• add docs on this add a beta channel #487

• add explanation to the aio interface how to switch channels and link to docs? (Only after domain was entered) add a beta channel #487

• Also show the channel in the aio interface then) show the current channel and add links to the changelogs #307

• we should also based on the channel point towards a changelog link: show the current channel and add links to the changelogs #307
  beta: https://github.com/nextcloud/all-in-one/releases
  stable/latest: https://github.com/nextcloud/all-in-one/releases/latest

• rework template for new releases done in template for new releases #180

• rework workflows to publish all container togethrr add promote-to-beta workflow and rework publishing workflows nextcloud-releases/all-in-one#10 add a beta channel #487