Review PR checklist in CONTRIBUTING.md

[aneta-petrova]

What changes are you introducing?

• Review PR checklist in the contribution doc for simplicity and clarity
• Drop a few items from the PR checklist that are obsolete or duplicate other parts of the contribution doc
• Splitting the PR checklist into two smaller checklist to make it a bit less overwhelming

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

Follow-up on #4281

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

N/A

Contributor 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 3.16/Katello 4.18 (Satellite 6.18)
• Foreman 3.15/Katello 4.17
• Foreman 3.14/Katello 4.16 (Satellite 6.17; orcharhino 7.4)
• Foreman 3.13/Katello 4.15 (EL9 only)
• Foreman 3.12/Katello 4.14 (Satellite 6.16; orcharhino 7.2 on EL9 only; orcharhino 7.3)
• 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)
• We do not accept PRs for Foreman older than 3.9.

[github-actions]

The PR preview for be05015 is available at theforeman-foreman-documentation-preview-pr-4291.surge.sh

No diff compared to the current base

show diff

[maximiliankolb]

LGTM!