Add information about the client-2 repository

[AkshayGadhaveRH]

Puppet agent 8 is provided via the client-2 repository as, the client repo is release neutral and cannot contain both Puppet agents 7 and 8.

Adding information about the client-2 repository where necessary.

JIRA:
https://issues.redhat.com/browse/SAT-28268
https://issues.redhat.com/browse/SAT-27092
https://issues.redhat.com/browse/SAT-28287

What changes are you introducing?

Why are you introducing these changes? (Explanation, links to references, issues, etc.)

Anything else to add? (Considerations, potential downsides, alternative solutions you have explored, etc.)

Checklists

• I am okay with my commits getting squashed when you merge this PR.
• I am familiar with the contributing guidelines.

Please cherry-pick my commits into:

• Foreman 3.15/Katello 4.17
• Foreman 3.14/Katello 4.16 (Satellite 6.17)
• Foreman 3.13/Katello 4.15 (EL9 only)
• Foreman 3.12/Katello 4.14 (Satellite 6.16; orcharhino 7.2 on EL9 only)
• Foreman 3.11/Katello 4.13 (orcharhino 6.11 on EL8 only; orcharhino 7.0 on EL8+EL9; orcharhino 7.1 with Leapp)
• Foreman 3.10/Katello 4.12
• Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8/6.9/6.10)
• Foreman 3.8/Katello 4.10
• Foreman 3.7/Katello 4.9 (Satellite 6.14)
• We do not accept PRs for Foreman older than 3.7.

[maximiliankolb]

@AkshayGadhaveRH Is this PR still relevant? If so, please rebase to HEAD of "master". If not, please close it.