[DOCS] Add documentation for ext:dashboard

Initial documentation covering basic editor, integrator and developer
tasks.

Resolves: #90340
Releases: master
Change-Id: Ib4917b8f618062ec2f6566415bee7fdf611a2dcc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63754


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