Add envelope_prob entry to rcParams (#138)

* added envelope_prob to rcParams

* added envelope_prob parameter to api docs

* run current docstub version

---------

Co-authored-by: Oriol (ProDesk) <oriol.abril.pla@gmail.com>

Author: NeerjaKasture

docs/source/api/index.md

@@ -222,6 +222,15 @@ which sometimes requires updating default values, for example to use new algorit
     The default probability of computed credible intervals. Its default value here
     is also a friendly reminder of the arbitrary nature of commonly values like 95%
 
+.. py:data:: stats.envelope_prob
+    :type: float
+    :value: 0.99
+
+    The default probability of envelopes used in diagnostic plots. The relatively high
+    default value reflects their role in highlighting systematic departures
+    from the model while reducing false positives due to random variation.
+
+
 .. py:data:: stats.round_to
     :type: int or str
     :value: "2g"

src/arviz_base/rcparams.py

@@ -336,6 +336,7 @@ def validate_iterable(value):
     "stats.module": ("base", _validate_stats_module),
     "stats.ci_kind": ("eti", _make_validate_choice({"eti", "hdi"})),
     "stats.ci_prob": (0.89, _validate_probability),
+    "stats.envelope_prob": (0.99, _validate_probability),
     "stats.round_to": ("2g", _validate_rounding),
     "stats.ic_pointwise": (True, _validate_boolean),
     "stats.ic_scale": (