DOC: Add experimental flag to Nested Hybrid Grid module (#441)

Since Nested Hybrid Grid is still experimental and the API will change as we experiment on it, we need to flag it so that user understand the risk.

No test as it's only documentation and temporarily printing out a warning.

Author: alifbe

docs/nestedhybridgrid.rst

@@ -1,6 +1,12 @@
 Nested Hybrid Grids
 ====================
 
+.. admonition:: 🧪 Experimental Feature
+   :class: warning
+
+   This module is currently experimental. It may undergo breaking changes 
+   in future versions without notice.
+
 .. contents:: Table of Contents
    :local:
    :depth: 2

src/fmu/tools/nestedhybridgrid/nestedhybrid.py

@@ -17,6 +17,7 @@
 from __future__ import annotations
 
 import logging
+import warnings
 from typing import TYPE_CHECKING
 
 import numpy as np
@@ -358,6 +359,11 @@ def create_nested_hybrid_grid(
         the coarse grid and *nnc_table* is a :class:`pandas.DataFrame` mapping
         mother cells to their connected refined cells.
     """
+    warnings.warn(
+        "create_nested_hybrid_grid is currently experimental. It may undergo "
+        "breaking changes in future versions without notice.",
+        FutureWarning,
+    )
     if any(r < 1 for r in refinement):
         raise ValueError(f"Refinement factors must be >= 1, got {refinement}")