[feature] Update the Casbin website to add ASF incubation compliance notice

[hsluoyz]

The Casbin official website should be updated to include some ASF-required incubation compliance items.

In particular, the footer should clearly state that the project is (Incubating).

Please align the website with the ASF incubation requirements for Casbin. You can refer to the Apache project page and similar incubating project websites for wording and presentation style.

Main requirement:

• Add “(Incubating)” to the footer declaration.
• Make it all Green

Suggested references:

• Casbin project page: https://whimsy.apache.org/pods/project/casbin
• Similar ASF incubating project example: https://whimsy.apache.org/pods/project/fesod
• Example website: https://fesod.apache.org/