Improve documentation for third-party frontends

[HeroicKatora]

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, most documentation currently lives only under 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:

• Additional guides for implementing custom frontends in the main frontends module
• 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.
• 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.

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