Add block_id example for GTFS Schedule

[Sergiodero]

There's a vote in the transit repo PR #386 to add a recommended presence to GTFS and include best practices on adding specific files and fields. Voting ends on 2023-07-31 at 23:59:59 UTC.

One point of discussion in this PR involved the inclusion of the May be provided for frequency-based trips text for trips.block_id field (see best practices). As a result of this discussion, adding a new use case in the GTFS.org example section was proposed to supplement the best practices text, illustrating the use of block_id with frequency-based trips.

The example shared in issue #227 can be used as a solid base to develop a new example, ideally one that also clarifies the appropriate use of block_id for loop routes (see best practices organized by case)