Improve documentation for third-party frontends

Project Improvement
======

It is currently unclear where to start with implementing bindings for other libraries. While the main module for such documentation should be [`frontends`][frontends], most documentation currently lives only under [`code_grant::frontend`][code_grant_frontend]. Additionally, the 'example' code given by the iron frontend is not extremely friendly as a guide, as it is missing additional reasoning and remarks and includes a bulk of iron specific overhead at the top.

Main features to improve are the following:
- [x] Additional guides for implementing custom frontends in the main `frontends` module
- [x] ~Add a simpler example 'software' frontend which exemplifies the main structure of any frontend and realizes the guide. A prototype for this is currently implemented for testing purposes under `code_grant::tests`.~ The rouille frontend does a good job here.
- [x] ~Improve the code style (structure, documentation, exports, maybe even item order) in `code_grant::frontend` so that the code is more self-documenting and to reduce overhead of implementing bindings.~ A module `frontends::dev` exports most types necessary to adapt a custom frontend.
- [ ] Provide better, informational links both between related code items as well as to external resources concerned with OAuth 2.0 in general.

Tracking pull request
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- [ ] A pull request does not yet exist

[frontends]: https://docs.rs/oxide-auth/0.2.0/oxide_auth/frontends/index.html
[code_grant_frontend]: https://docs.rs/oxide-auth/0.2.0/oxide_auth/code_grant/frontend/index.html