Add GTFS-Flex data examples to gtfs.org

[tzujenchanmbd]

As GTFS-Flex is moving forward to a new official spec extension, there is need to create a page like fares-v2 to better communicate how to model different use cases. In addition, as MobilityData/gtfs-flex #76mentioned, since flex routing can be relatively fuzzy (the nature of no fixed schedule), it's important to reduce miscommunication between producers and consumers.

We will create data example page on gtfs.org once the proposal being adopted. Please share any useful use cases / edge cases here so that we can illustrate how to model them appropriately.