Improve documentation about upgrading from CKEditor 4 to 5 prior to upgrading 2.10.0+

[joeparsons]

Problem/Motivation

We've heard some reports of issues encountered by site owners when upgrading sites to AZQS 2.10.x because they had missed the needed steps to upgrade their editor configs to CKEditor 5 and uninstall the ckeditor (4) module prior to upgrading and 2.10.0 is when we removed the contrib ckeditor (4) module from AZQS.

Describe the bug

If a site's editor configs haven't been upgraded to CKEditor 5 and the ckeditor (4) module hasn't been uninstalled prior to upgrading a site to 2.10.0+, trying to edit any content using the CKEditor 4 editors results in fatal errors / white screens:

"Drupal\Component\Plugin\Exception\PluginNotFoundException: The "ckeditor" plugin does not exist. Valid plugin IDs for Drupal\editor\Plugin\EditorManager are: in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 53 of /code/web/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php)."

Proposed resolution

• Add a CKEditor 4 > 5 upgrade notes wiki page
• Link to new wiki page in 2.8.0, 2.9.0 and 2.10.0 release notes

[joeparsons]

Draft documentation:

CKEditor 4 was removed from Drupal core as of Drupal 10.0.0. We added the CKEditor (4) contrib module to AZQS 2.7.0 as part of our upgrade to D10.

We made the core ckeditor5 module a default module for AZQS as of 2.8.0 but didn't remove the ckeditor (4) module until AZQS 2.10.0 to ensure that site admins had sufficient time to verify their text editor configurations were CKEditor 5 ready and time to uninstall the CKEditor (4) module before it was removed from AZQS. We also intentionally chose to not automate the uninstall of the ckeditor (4) module in case site admins had their own editor configurations or had made any modifications to the default AZQS editor configs. For our Arizona Sites, the upgrade of the text editor configurations from CKEditor 4 to 5 was fairly automatic as part of the AZQS 2.8.0 minor release upgrade.

It should be possible to still add the ckeditor (4) contrib module back to the site as a site-specific composer dependency which may fix the problems being seen and also should make it possible to ensure that the text editor config(s) on the site have been updated to CKeditor 5 and then ensure the ckeditor (4) module is properly uninstalled.

The ckeditor (4) module has been marked unsupported/obsolete on drupal.org due to known security issues with CKEditor 4 but it should still be possible to add the last release to the site with composer: https://www.drupal.org/project/ckeditor/releases/1.0.2