document how to mirror Ubuntu ESM packages

[stdevel]

Some hints

Consider adding an entry to the CHANGELOG.md file in the toplevel directory. Cosmetic changes such as fixing typos do not need log entries (nevertheless it is important to fix typos, etc.)! In the manager-4.3, the hidden .changelog file with a leading dot is still in use.

Add Description, Target branches, and related Links below the following section titles.

In the Description, just enter a summary of why you created this PR (and, if available, add any relevant diagram).

Description

This PR documents how to mirror Ubuntu Pro packages, see also issue #2971

Target branches

• Which product version this PR applies to (Uyuni, SUMA 4.3, SUMA MU X.Y.Z, or SUMA development version). This information can be helpful if ifeval statements are needed to publish it for certain products only.
• Does this PR need to be backported? If yes, create an issue for tracking it and add the link to this PR.
• Whenever possible, cross-reference each backport PR here, so that all backports can be easily accessed from the description.

Backport targets (edit as needed):

• master
• 4.3

Links

• This PR tracks issue #2971