[DOCS] Add migration chapter to dashboard docs

To ease the access for extension developers the migration of chart
widget configuration is described in the dashboard documentation.

Related: #99150
Resolves: #99483
Resolves: https://github.com/TYPO3-Documentation/Changelog-To-Doc/issues/268
Releases: main
Change-Id: I6d94fc02de16a374e942cf765b3f1c861fd25e71
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77282


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Lina Wolf <112@linawolf.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Lina Wolf <112@linawolf.de>