Improve example documentation

# Project Improvement

The lack of comments and the complexity in the setup leads to bad understanding of the examples, especially without prior knowledge of OAuth2's inner workings. In particular:

- It's unclear which portions of the examples belong to the server and must be provided.
- Which are purely for the sake of the example client. This being an increased concern for `rocket` where we _must_ use the same server as the library only allows 'launching one Rocket'.
- The example structures differ quite a bit. (See also #111)

## Comment

Original report (through another channel) also says this:

> what was unclear in the examples what are the required parts of example for the server side of the authentication in particular the Client Fairing impl and whats just for the client side in the two files names rocket.rs, something that would be helpful would just adding comments where they are missing in the those two files

# Tracking pull request

- [ ] A pull request does not yet exist

Look at the example files, likely in an examples/ directory, focusing on rocket.rs and any other server/client examples. Identify which parts are server-side (like Client Fairing impl) and which are client-side. Add clarifying comments to separate these responsibilities and explain the setup, especially for the rocket example where only one server can be launched.

Rédigé par le modèle d'indexation à partir du texte de l'issue.