make a book to demonstrate features

[0x8f701]

rhai made an online book which looks really useful. https://schungx.github.io/rhai, users will be able to print to file for offline usage.

[PsiACE]

mdbook is a good choice, and it is very convenient to deploy.

So what kind of content should we provide? Some simple examples or a real-world application.

[hsluoyz]

Why don't use our casbin website docs: https://casbin.org/docs/en/overview ?

[0x8f701]

@hsluoyz casbin.org can be used to show APIs. A book can be used to show practical rust examples of RBAC (with/without domain)，ABAC，Adapters，Cache，Watcher，Logging etc and also how does it work，we will have more chances to show things without caring site looking.

[0x8f701]

@PsiACE This is low priority. Let's focus on raft first.

[hsluoyz]

OK.