docs: add documentation for rfl::Commented feature

- Created docs/commented.md with examples for YAML and XML.
- Highlighted that comments are write-only and ignored during deserialization.
- Updated mkdocs.yaml to include rfl::Commented in the navigation.
- Added rfl::Commented to the feature overview in README.md.

Co-authored-by: liuzicheng1987 <19538706+liuzicheng1987@users.noreply.github.com>

Author: google-labs-jules[bot]

docs/commented.md

@@ -2,7 +2,7 @@
 
 The `rfl::Commented<T>` wrapper allows you to add comments to fields in your structs. These comments are then serialized in formats that support them, such as YAML and XML.
 
-Note that `rfl::Commented` is currently unsupported by formats that do not have a standard way of representing comments, such as JSON.
+Note that `rfl::Commented` is currently unsupported by formats that do not have a standard way of representing comments, such as JSON. Also note that comments are **write-only**: they are ignored during deserialization.
 
 ## Example (YAML)