[TASK] Clean up of ext:dashboard

Before the final release of the dashboard some clean up is done.
Among others following tasks are performed:

* Add missing PHPDocs
* Remove unused methods
* Remove superfluous properties
* Unify namespace format
* Fix some typos
* Fix some formatting
* Make closures static
* NoDashboardAvailableException is removed as it is never be thrown
* Extend documentation for missing widget groups

Resolves: #90524
Releases: master
Change-Id: Ia0f7d71def358bf0ee8ab42d97f012c14cdede1c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63407


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>