[Direct Edit] Clean up the docs when the deprecated legacy path is removed

[markus-moser]

2026.3 deprecated the Direct Edit bundle's own Mercure hub client and the controller trait built on it, for removal in 2027.1 (pimcore/direct-edit#137). Both are still documented, because until the removal they still work and Portal Engine's Classic frontend still depends on them. When the code goes, this documentation has to go with it, or the docs will describe a path that no longer exists.

All paths below are in pimcore/direct-edit.

What to delete

• doc/01_Installation/04_Mercure_Setup.md: delete ## 3 One Extra Setting for Legacy Frontends (deprecated). It is the tail of the file, so it is one contiguous cut. Then remove what still points at it from above: the "three setup steps" sentence in the intro, the anonymous-subscriber clause in step 1, the hub-URL-shape clause after the nginx snippet, and the frontmatter description.
• doc/10_Integrate_into_Custom_Application.md: delete ## Legacy: FileEditControllerTrait, also the tail of the file, and the parenthetical in step 4 about the legacy topic still being published.
• doc/01_Installation/README.md: under Bundle Configuration, the mercure_settings deprecation sentence and the four mercure_settings.* rows of the table; under Mercure Setup, the trailing clause about the extra setting.
• README.md: the Mercure Setup line in the documentation index.
• doc/01_Installation/05_Upgrade_notes.md: add the 2027.1 removal section. The 2026.3 deprecation block stays as history.

Worth knowing when doing it

• Step 3 deep-links to the trait section of the custom-application page. Delete both in the same commit so the link does not dangle.
• If bin/mercure is dropped in the same release, ### Shipped Mercure Binary in step 1 goes with it.
• Portal Engine's doc/05_Administration_of_Portals/14_Direct_Edit.md links into this page from another repo, and its own legacy frontend is removed on the same schedule.

Code removed in the same release

For cross-checking that no documentation is left behind: Controller\FileEditControllerTrait, Service\ModalRenderService, Model\Response\DirectEditModal, Service\PublishService, Service\MercureUrlService, Model\Mercure\Subscription, Service\Permission\PimcoreBackendPermissionService, the legacy topic http://www.pimcore.com/direct-edit/client-upload/user/<userId> with the methods that publish it on ClientCommunicationService, the pimcore_direct_edit.mercure_settings configuration and the parameters.mercure.hub.* keys.