[tech] Improve ignored doc-tests

[pbougue]

After #782 (comment)
Some doc-tests are ignored, which is a shame.
The main point is that we want to avoid a heavy (compile-time) dependency for the gain (reqwest was removed for that, and deps on async frameworks are heavy).

We have 4 options on that :

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/ or https://users.rust-lang.org/t/lightweight-alternative-for-reqwest/33601/2 for lighter http clients (ureq, http_req).
   Or add reqwest (with some internal TLS support to avoid system deps?) back to check if it's OK for compile times.
   a. add dev-deps and remove ignore in tests
   b. add complete deps (and real url-read fn for convenience as it's the most common use case)

I will probably have a try on option 2.a (maybe not short-term), but if anyone wants to try some of the options listed or others, please do!

Also, any feedback from users outside Kisio Digital (ex-CanalTP) is more than welcome on the usefulness of this 🙏