docs(ref): Link to docs.rs metadata table

This gets us part of the cross-linking described in #7164

Author: epage

src/doc/src/reference/manifest.md

@@ -511,6 +511,9 @@ package-name = "my-awesome-android-app"
 assets = "path/to/static"
 ```
 
+First-party `package.metadata` tables:
+- [docs.rs](https://docs.rs/about/metadata).
+
 There is a similar table at the workspace level at
 [`workspace.metadata`][workspace-metadata]. While cargo does not specify a
 format for the content of either of these tables, it is suggested that