Added doc page for StringContainsValidator.

Refs #20832

docs/source/api/python/mantid/kernel/StringContainsValidator.rst

+=======================
+StringContainsValidator
+=======================
+
+This a python binding to the C++ class Mantid::Kernel::StringContainsValidator.
+
+*bases:* :py:obj:`mantid.kernel.IValidator`
+
+.. module:`mantid.kernel`
+
+.. autoclass:: mantid.kernel.StringContainsValidator
+    :members:
+    :undoc-members:
+    :inherited-members: