[TASK] Rename GeneralUtility in Documentation sysext

Rename the GeneralUtility class in the Documentation sysext to avoid
confusion with GeneralUtility used in the Core sysext.

Resolves: #57197
Releases: 6.2
Change-Id: Iff44c713307c86840d60bc9ef5aee67ae9741fa9
Reviewed-on: https://review.typo3.org/28655
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe