[feature] Recipe attribute for library documentation url

It would be nice to have an optional attribute for recipes to add a URL to the documentation of the library.

As `url` attribute points to the recipe origin repo (in the case of ConanCenter it is always `conan-center-index` repo) and `homepage` usually points to the library's repo or project web page, we are missing a direct link to the library documentation that could be:

- Specific page in the homepage of the project.
- Wiki section of library's repository.
- Specific folder inside library's repository (many libraries have their documentation next to the source code inside folders like `/docs` of `/examples`).
- ...

This would be nice information to have in order to get library code reference quickly via recipe inspection from Conan command line or from a future redesign of the ConanCenter webpage.