Remove system purpose addons references

[chris1984]

What changes are you introducing?

We removed system purpose addons in Katello, this is a PR to remove and references from the docs.

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

We removed addons here Katello/katello#11196

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

N/A

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 3.13/Katello 4.15 (EL9 only)
• Foreman 3.12/Katello 4.14 (Satellite 6.16)
• Foreman 3.11/Katello 4.13 (orcharhino 6.11 on EL8 only; orcharhino 7.0 on EL8+EL9)
• 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.

[github-actions]

The PR preview for e102bb9 is available at theforeman-foreman-documentation-preview-pr-3597.surge.sh

The following output files are affected by this PR:

• Managing_Content/index-katello.html
• Managing_Content/index-orcharhino.html
• Managing_Content/index-satellite.html
• Managing_Hosts/index-satellite.html

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