[TASK] Add examples to ext:backend ViewHelpers

Following ViewHelpers now also feature proper examples:
* AvatarViewHelper
* ThumbnailViewHelper

As the ViewHelpers are not declared as `@internal` custom extension
can use them for e.g. backend modules. Therefore they now contain
examples about the usage.

Resolves: #89629
Releases: master, 9.5
Change-Id: Iff9a4f6e5a0cc02799f093af0a5abf91eff9fcf3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62266


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Daniel Siepmann <coding@daniel-siepmann.de>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>