Document how to populate foreman-contributors.adoc

[ekohl]

The script lives in theforeman.org repo because that's where it has historically lived. That repository also contains the mailmap to ensure multiple identities (different spelling or email) are mapped to a single name.

Something to consider: this doesn't update Katello and the script isn't really suited for that right now. I debated merging the two into one big list of contributors. Simply include Katello contributors as Foreman contributors.

Another note: we need to update the procedures to reuse this:
https://github.com/theforeman/theforeman-rel-eng/blob/ebfe21978af75a30a96d3716ddbd403d7b53aac3/procedures/foreman/release.md.erb#L102
https://github.com/theforeman/theforeman-rel-eng/blob/ebfe21978af75a30a96d3716ddbd403d7b53aac3/procedures/katello/release.md.erb#L35

Fixes #1767

• 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 3.11/Katello 4.13
• Foreman 3.10/Katello 4.12
• Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8/6.9)
• Foreman 3.8/Katello 4.10
• Foreman 3.7/Katello 4.9 (Satellite 6.14)
• Foreman 3.6/Katello 4.8
• Foreman 3.5/Katello 4.7 (Satellite 6.13; orcharhino 6.6/6.7)
• We do not accept PRs for Foreman older than 3.5.

[github-actions]

The PR preview for d21a5cf is available at theforeman-foreman-documentation-preview-pr-3202.surge.sh

The following output files are affected by this PR:

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[Lennonka]

Why don't we move the script to this repo?

[ekohl]

It's not just the script, but we also have a mailmap there that's used. Right now it's now used in 2 places and until we fully migrate away from the website, I think it's best to keep it there.