[TASK] Load documentation extension by default

The documentation extension should be loaded by default for new
websites and should be suggested in Install Tool when upgrading.

Resolves: #50536
Releases: 6.2
Change-Id: I97755b96cf0ccce849e986650d5deca37cde294a
Reviewed-on: https://review.typo3.org/22668
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters