Documentation around Segmentation

[sgiehl]

Our documentation currently leaks some details around Segmentation. There is only https://developer.matomo.org/guides/dimensions#segmentation, which mentions how new segments can be defined for a dimension.

We should add at least some documentation around Dimension::configureSegments and the special methods of Piwik\Plugin\Segment that can be used to define more complex segments. Like setSqlFilter, setSqlFilterValue or setUnionOfSegments.

In addition we should also mention that segments can be manipulated outside of dimensions by using the hooks of Piwik\Segment\SegmentsList: Segment.addSegments and Segment.filterSegment

Not sure where to add that, maybe https://developer.matomo.org/guides/segments would be the correct place. We could add a new section like How to provide new Segments or similar.

follow-up to matomo-org/matomo#18953