[DOCS] Add dashboard widget registration example via Services.php

Explain how to register dashboard widgets via Services.php instead of
Services.yaml.
This might be necessary if a widget is provided under certain
circumstances, e.g. if a specific extension is enabled.

Resolves: #91159
Releases: master
Change-Id: Ic16e79f478b6da446ff1854979c3c3a822674d1e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64296


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Richard Haeser <richard@maxserv.com>