[tech] [doc] test doc examples

[pbougue]

After https://github.com/CanalTP/transit_model/pull/780/files#r629252682
Renaming the zip files after their move will be required (and a pain point).
So maybe not worth keeping all this, but tested what was missing to activate all doc-test 🤷

[antoine-de]

fine for me, a real doc test is always better, but reqwest was removed previously because of the impact on the build time

[pbougue]

4 options:

1. No http-client deps
   a. let it as-is (ignored)
   b. use file read + comment that it could be reqwest
2. We can check https://blog.logrocket.com/the-state-of-rust-http-clients/ for light http clients (ureq, http_req).
   Or add reqwest back to check if it's OK for compile times.
   a. add dev-deps
   b. add complete deps (and real url-read)