Gateway docs need to be updated to reflect supported features

Checklist

• My issue is specific & actionable.
• I am not suggesting a protocol enhancement.
• I have searched on the issue tracker for my issue.

Description

https://github.com/ipfs/kubo/blob/master/docs/gateway.md For convenience, the gateway (mostly) acts like a normal web-server when serving a directory:

If the directory contains an index.html file: If the path does not end in a /, append a / and redirect. This helps avoid serving duplicate content from different paths.† Otherwise, serve the index.html file. Dynamically build and serve a listing of the contents of the directory.

Among the above functions, a new page redirection function is added for directories without index.html, such as redirecting to ipfs.io. No need to directly build directory listing contents