Add install docs for drupal 9 now that the demo site shows it works

ping @MikeyMJCO

This is mostly a carbon copy of D8 docs but with the version changed and also added a note that we have only verified from 5.28 onwards. Maybe need a note in there about cv version maybe?

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• e52b9064 - Add install docs for drupal 9 now that the demo site shows it works

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(1) Guess: this has been sitting for a bit because it adds an entire CMS/chapter. There's a higher bar to verifying the procedure.

(2) I'm trying to do some multi-file cleanups. I don't want them to make this stale, and I don't want this MR to block those... it would be easier to do those bits of cleanup with D9 files at the same time as the others.

(3) Therefore, I've rebased this and squashed-out the change to mkdocs.yml (and will use that commit as a reference-point in my own branch). Perhaps we could merge this MR as inert/wip file? Or just add a title/header to indicate it's status?

mentioned in merge request !14 (merged)

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• 45583ad5 - docs/drupal9 - Add warning to the top about document status

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So, running mkdocs via docker, the behavior with an unlisted page is that:

1. It's not listed. :)
2. It's available if you navigate to the corresponding URL.

I'm not certain if the live publication (post-merge) would behave the same way. It seems the most likely outcomes are either (a) it will do the same or (b) it will ignore the unreferenced file. Either seems acceptable for the moment.

(I would be concerned if there were dangling references, because that has produced errors in the past. But rgrep drupal9 doesn't find any.)

Merging.

mentioned in commit f34719c5

merged

Confirmed that the live publication behaves the same way -- you can view https://docs.civicrm.org/installation/en/latest/drupal9/ if you know the URL, but it doesn't show up in the menu.