Create CEL API Reference and link it from README and sitemap.

Add a unified API Reference document for CEL macros, operators, and functions
under third_party/cel/spec/docs/api-reference.md. This document includes a
support matrix for Go, C++, Java, Python, and C (runtime-only) stacks, with
granular introduction release versions for each individual function.

Key changes:
- Added 'Stack Versions' section listing documented release versions of CEL.
- Added 'Network Library' section documenting the Go-only IP/CIDR functions.
- Granularized compatibility tables to document the exact introduction release
  version of each individual function instead of the library addition.
- Split bundled function rows (Bitwise, Rounding, Classification, first/last,
  Lower/Upper) into individual rows with independent versioning.
- Corrected C++ and Java compatibility for `json.encode` (marked as unsupported).
- Updated Python compatibility checkmarks and enablement notes for Lists,
  Sets, Two-Variable Comprehensions, and Regex (supported via config).
- Added Python unit tests verifying standard extensions (Lists, Sets,
  Two-Var Comprehensions, Regex) can be enabled via config.
- Revised the LLM prompt for API doc generation to mirror the final doc layout.

PiperOrigin-RevId: 952408595

Author: TristonianJones

cel_expr_python/cel_env_test.py

@@ -490,6 +490,34 @@ def test_config_extensions(self):
     res = env.compile("hello('World')").eval()
     self.assertEqual(res.value(), "Hello, World!")
 
+  def test_standard_extensions_via_config(self):
+    config: cel.EnvConfig = cel.NewEnvConfigFromYaml("""
+      extensions:
+        - name: lists
+        - name: sets
+        - name: two-var-comprehensions
+        - name: optional
+        - name: regex
+      """)
+    env: cel.Env = cel.NewEnv(config=config)
+
+    # Test lists extension (e.g., reverse)
+    res = env.compile("[1, 2].reverse()").eval()
+    self.assertEqual(res.plain_value(), [2, 1])
+
+    # Test sets extension (e.g., intersects)
+    res = env.compile("sets.intersects([1, 2], [2, 3])").eval()
+    self.assertEqual(res.value(), True)
+
+    # Test two-var-comprehensions (e.g., all)
+    res = env.compile("[1, 2].all(i, v, v > 0)").eval()
+    self.assertEqual(res.value(), True)
+
+    # Test regex extension (e.g., replace)
+    expr = r"regex.replace('123-456', r'(\d+)-(\d+)', r'\2-\1')"
+    res = env.compile(expr).eval()
+    self.assertEqual(res.value(), "456-123")
+
   def test_config_extension_override_same_version(self):
     config: cel.EnvConfig = cel.NewEnvConfigFromYaml("""
       extensions: