Prevent the automatic creation of Wagtail Page Aliases in other locales

[stevejalim]

This changeset deliberately removes the after_create_page hook from wagtail_localize.synctree to avoid the default behaviour where adding a page in a locale creates Aliases in all other locales which then serve the soure locale's content but under a different locale path (eg /en-US/test-page/'s content appears on /fr/test-page/ because of the alias)

See changes to cms.rst in this changeset for more details

• I used an AI to write some of this code: specifically, the approach to patching out the wagtail hook, but it seems to work well and I've added a test to protect against regressions

Issue / Bugzilla link

Resolves #15063

Testing

• a[ ] On main, create a new page in the CMS, publish it.
• Confirm you can see that page also created in all other locales (ping @stevejalim if you don't get this default behaviour).
• If you edit one of those versions it will say it's an Alias and needs to be made into a page to edit it.
• On this branch, create another new page and publish it.
• Confirm you no longer have a crop of Aliases made in the other locales.
• In the kebab/context menu for the page you made, click Translate this page and select one or two locales. Do not click the buttong related to Smartling and then submit/save
• Confirm you can find versions of the source page in the page tree for the other locales. They will default to the en-US content for now, but you can edit them and publish new "localized" content in them
• Stretch test: edit the source page again and republish. Use the kebab menu to sync translated content again. This should not error. View a localized version of the page and confirm that the source strings for the page have been synced over, and that a new translation is needed for that updated content.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.89%. Comparing base (4e7a269) to head (fb7d806).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15066      +/-   ##
==========================================
+ Coverage   77.85%   77.89%   +0.03%     
==========================================
  Files         162      163       +1     
  Lines        8450     8463      +13     
==========================================
+ Hits         6579     6592      +13     
  Misses       1871     1871              

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[stevejalim]

See comment on #15063