[BUGFIX] Cleanup EXT:cshmanual

* Removed require_once which is deprecated.
* Introduced a use-statement for GeneralUtility
* Removed TYPO3_MOD_PATH
* Links generated by make_seeAlso() were double htmlspecialchars
  encoded after security patch https://review.typo3.org/27636

Resolves: #56826
Related: #55509
Releases: 6.2
Change-Id: I8effc7c6bf9828dde4f1c69754b207864b3122ba
Reviewed-on: https://review.typo3.org/28303
Reviewed-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring