[DOCS] Update finisher documentation

Update the finisher documentation to accomodate changes
made within the deprecation notices:

- remove mixins from finisher backend configuration
- add note to explain "mixins" have been removed in TYPO3 v11
- add a header as well as a removeButton in Backend UI config
  to restore functionality
- add a reminder to clear all caches to help newbies to see their
  changes while testing

Releases: main
Resolves: #99691
Change-Id: Icad673f1e23f595b6ffbae9afef8d01425bd1ea2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77556


Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>