[BUGFIX] Cannot fetch documentation on Windows

TYPO3's internal methods expect forward slashes when computing
absolute filenames.

Fixes: #52745
Releases: 6.2
Change-Id: I00df69f280fa8ddfd59ae1a41c1919003b74362a
Reviewed-on: https://review.typo3.org/24673
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Wouter Wolters
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes