Add experimental feature warning to stdlib module and language reference

Author: HT154

docs/modules/language-reference/pages/index.adoc

@@ -5951,6 +5951,14 @@ To fully set up mirroring, an HTTP(s) server will need be running, and which mir
 [[references]]
 === References
 
+[WARNING]
+.Experimenal Feature
+====
+The API and semantics of references are subject to change in a future release.
+The Pkl team is soliciting feedback from authors of libraries considering adopting references.
+For questions and feedback, please reach out via https://github.com/apple/pkl/discussions[GitHub Discussions] or https://github.com/apple/pkl/issues/new[create an issue]. 
+====
+
 pkldoc:Reference[pkl:ref]s (provided by the `pkl.ref` module) are an advanced API design tool that serves use cases where configuration must refer to values that are not known at evaluation time.
 This is common in task-based execution systems like CI platforms and Infrastructure as Code (IaC) tools.
 Today, many such systems encode this kind of reference using strings containing an expression language; this approach makes it difficult to write and debug such references, as target systems may not know where in the Pkl source an error occurred.

stdlib/ref.pkl

@@ -20,11 +20,23 @@
 /// domain-specific references to values that may not be known during evaluation.
 /// They are particularly suited for configuring execution systems where tasks have well-typed
 /// inputs and output.
+///
+/// WARNING: The API and semantics of references are subject to change in a future release.
+/// The Pkl team is soliciting feedback from authors of libraries considering adopting references.
+/// For questions and feedback, please reach out via
+/// [GitHub Discussions](https://github.com/apple/pkl/discussions) or
+/// [create an issue](https://github.com/apple/pkl/issues/new).
 @ModuleInfo { minPklVersion = "0.32.0" }
 module pkl.ref
 
 /// Creates a deferred [Reference] to a value of type [class] in the given [domain].
 ///
+/// WARNING: The API and semantics of references are subject to change in a future release.
+/// The Pkl team is soliciting feedback from authors of libraries considering adopting references.
+/// For questions and feedback, please reach out via
+/// [GitHub Discussions](https://github.com/apple/pkl/discussions) or
+/// [create an issue](https://github.com/apple/pkl/issues/new).
+///
 /// References may only be constructed for single, non-generic class types.
 /// To create a reference to other types (generic classes, union, nullable, constrained,
 /// typealiases, etc.), it may be necessary to create a wrapper class with a property of the desired
@@ -48,6 +60,12 @@ external const function Reference<D, T>(
 /// References are an advanced API design tool that enables library authors to express
 /// domain-specific references to values that may not be known during evaluation.
 ///
+/// WARNING: The API and semantics of references are subject to change in a future release.
+/// The Pkl team is soliciting feedback from authors of libraries considering adopting references.
+/// For questions and feedback, please reach out via
+/// [GitHub Discussions](https://github.com/apple/pkl/discussions) or
+/// [create an issue](https://github.com/apple/pkl/issues/new).
+///
 /// Instances are created with the [Reference()] constructor method.
 /// References consist of four parts:
 /// * [Domain]: determines which references are compatible and how references are rendered