docs: guard pytestconfig.cache example when cacheprovider is disabled (#14254)

Update the `config.cache` example in `doc/en/how-to/cache.rst` to safely handle cases where the `cacheprovider` plugin is disabled.

Closes #14148

Author: mitre88

changelog/14148.doc.rst

@@ -0,0 +1,2 @@
+Documented a safe ``pytestconfig.cache`` access pattern when the
+``cacheprovider`` plugin is disabled.

doc/en/how-to/cache.rst

@@ -214,11 +214,18 @@ across pytest invocations:
 
     @pytest.fixture
     def mydata(pytestconfig):
-        val = pytestconfig.cache.get("example/value", None)
+        cache = getattr(pytestconfig, "cache", None)
+        if cache is None:
+            # pytestconfig not having the cache attribute means the
+            # cache plugin is disabled.
+            expensive_computation()
+            return 42
+
+        val = cache.get("example/value", None)
         if val is None:
             expensive_computation()
             val = 42
-            pytestconfig.cache.set("example/value", val)
+            cache.set("example/value", val)
         return val