[TASK] Add information on how to create custom workspaces

The documentation had no information where to create custom workspaces in the backend.
That information has been added.

Releases: master, 9.5
Resolves: #88378
Change-Id: I8bd6d7ad2ac291e3576b89d39f9698ce16a5572c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60772


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>