Gateway docs need to be updated to reflect supported features

### Checklist

- [X] My issue is specific & actionable.
- [X] I am not suggesting a protocol enhancement.
- [X] I have searched on the [issue tracker](https://github.com/ipfs/kubo/issues?q=is%3Aissue) 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

Start with docs/gateway.md and compare its directory-serving description with the behaviors listed in the issue. Update the documentation so supported directory handling, including index.html behavior and page redirection, is accurately described; completion means the gateway feature summary no longer claims unsupported directory listings.

Written by the indexing model from the issue text.