[TASK] Migrate backport and documentation workflows to shared reusable workflows

[CybotTM]

Summary

Migrates both workflow files to shared reusable workflows from
TYPO3-Documentation/.github.

Changes

Workflow	Before	After
backport.yml	22 lines inline	reusable-backport.yml (5 lines)
documentation.yml	15 lines inline	reusable-test-documentation.yml (8 lines)

Benefits

• Single point of maintenance — action version updates happen centrally
• Consistent configuration — all documentation repos test rendering the same way
• No allow-list concerns — .github workflows are automatically trusted

Dependencies and merge order

• Depends on .github#4 (backport label_pattern fix)
• Depends on .github#2 (adds reusable-test-documentation.yml)

[garvinhicking]

(Please note this is a POC we should not merge as-is, because we'd first have to import the referenced repo into our own maintenance scope. Thanks a lot, this is very helpful for discussion!)

[CybotTM]

Updated for merged TYPO3-Documentation/t3docs-ci-deploy#56