Document redirects from request_filter

[ting-hong-shieh]

Closes #498.

This revisits the documentation change proposed in #613 on current main and adds a runnable example.

Summary

• Clarify that returning Ok(true) and setting end_of_stream do not add HTTP message framing.
• Show a bodyless 302 response with Content-Length: 0 in the user guide.
• Add the same redirect path to the compiled gateway example, including a curl command.
• Document session.set_keepalive(None) as the alternative when the connection should close.

Testing

• cargo fmt --all -- --check
• cargo check -p pingora-proxy --example gateway
• cargo +1.85.0 check -p pingora-proxy --example gateway
• cargo test -p pingora-proxy --doc
• cargo clippy -p pingora-proxy --example gateway -- -D warnings
• RUSTDOCFLAGS='-D rustdoc::broken_intra_doc_links' cargo doc -p pingora-proxy --no-deps
• Ran the gateway example and requested /redirect over HTTP/1.1. Curl completed immediately with 302 Found, Location: /family/, Content-Length: 0, and an empty body while keeping the connection reusable.