Add v4 migration rehearsal tutorial

[nscuro]

To make it easier to v4 users to test with a local stack and provide feedback for the migration procedure.

[Copilot]

Pull request overview

Adds a new tutorial to help operators rehearse the v4→v5 migration against a local v5 sandbox stack, and wires it into the documentation site/navigation.

Changes:

• Add a new “Rehearsing the v4 to v5 migration” tutorial and link it from the migration guide and tutorials nav.
• Enable MkDocs Material tab linking to support tabbed snippets in the new tutorial.
• Minor docs housekeeping: update tech stack reference (Flyway), adjust context inventory text, and extend Vale vocabulary.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
mkdocs.yml	Enables content.tabs.link to support linked tabbed content in docs.
existing-docs/README.md	Removes prior placeholder README for the existing-docs/ symlink target.
existing-docs/.gitkeep	Keeps existing-docs/ tracked as an (otherwise) empty directory.
docs/tutorials/rehearsing-the-v4-migration.md	New end-to-end sandbox rehearsal tutorial for v4→v5 migration using the migrator container.
docs/tutorials/.pages	Adds the new rehearsal tutorial to the tutorials navigation.
docs/guides/administration/migrating-from-v4.md	Links to the new rehearsal tutorial and updates the migrator image name in the alias example.
context/source-inventory.md	Removes the “Existing Docs Accuracy” section from the source inventory context doc.
context/briefing.md	Updates tech stack wording from Liquibase to Flyway.
.vale/styles/config/vocabularies/DependencyTrack/accept.txt	Adds “cutover” to accepted vocabulary.

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