[roll] Roll fuchsia [fidl][docs] Standardize canonical examples

This is a pretty rote CL, cleaning up some inconsistencies in the
`fidl/examples/new` directories two existing canonical example series:
key_value_store and canvas. Things that have changed:

* No more references to Dart, since we not doing those canonical
  examples for the time being.
* Some cleaned up copy here and there.
* There was previously an inconsistent casing discipline between
  snake_case and kebab-case (ex: we had key-value-store and
  add_read_item variants). This has been fixed in favor of snake_case.
* s/case study/example/g
* TODOs are more regular, and always trimmed using a region_tag.
* Plus a few other small loose ends.

The utility of these change is in laying the ground work for the
scaffold generating script. We want the output to be very uniform, and
for parts that the script will later automatically replace (like the
TODO.md files) to be very consistently shaped.

Test: fx test examples_fidl
Original-Bug: 111614
Original-Bug: 111279
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/736643
Original-Revision: f8af29c868211e675f43ca6576678e9338a75837
GitOrigin-RevId: 47ec8c35eeee741a505087ed703286b266d2061a
Change-Id: I273ba9c6dc4ec43a20db512e2e557e7a674edec9