Update air-gapped install docs wrt ARM64

[kilfoyle]

Apparently ARM64 images aren't available for air-gapped installs. This adds a note to that effect on the Air-gapped environments page.

@cmacknz I'm just going by what was reported in Slack, but please let me know if that's not correct.

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[mrodm]

It looks good, specially for all the older stack versions <=8.14.

For 8.15 and onwards (main branch), we're trying to add support for multi-arch support for this two architectures: amd64 and arm64.
We hope this to be ready next week before 8.15 release.

If this is urgent, maybe it could be applied this change just for branch 8.14. WDYT @kilfoyle ?

cc @jsoriano

[kilfoyle]

@mrodm Certainly! I'll see if I can rebase this PR to merge only onto the 8.14 branch.

[mergify]

This pull request is now in conflict. Could you fix it @kilfoyle? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch <remote-repo>
git checkout -b air-gapped-arm <remote-repo>/air-gapped-arm
git merge <remote-repo>/main
git push <remote-repo> air-gapped-arm

[kilfoyle]

If this is urgent, maybe it could be applied this change just for branch 8.14. WDYT @kilfoyle ?

Thanks for the suggestion Mario! I've rebased this PR so it'll merge only into the 8.14 release.

[mrodm]

👍

[kilfoyle]

@mrodm The CI checks are failing unrelated to my docs change, so I've just opened a new PR with the same change. Would you mind giving that one a thumbs up if it looks okay? #1221

[kilfoyle]

Closing in favour of #1221