alternative workaround for i18n

This reference is pulled in from wtforms code because we subclass some
fields.

So let's point at their documentation for internationalisation to deal
with the warning that sphinx is flagging.

./git/flask-admin/flask_admin/contrib/sqla/fields.py:docstring of flask_admin.contrib.sqla.fields.QuerySelectField:64: WARNING: unknown document: 'i18n'
./git/flask-admin/flask_admin/contrib/sqla/fields.py:docstring of flask_admin.contrib.sqla.fields.QuerySelectMultipleField:43: WARNING: unknown document: 'i18n'
./git/flask-admin/flask_admin/contrib/sqla/fields.py:docstring of flask_admin.contrib.sqla.fields.CheckboxListField:55: WARNING: unknown document: 'i18n'
./git/flask-admin/flask_admin/form/fields.py:docstring of flask_admin.form.fields.Select2Field:41: WARNING: unknown document: 'i18n'

Author: samuelhwilliams

doc/api/i18n.rst

@@ -0,0 +1,4 @@
+Internalization
+===============
+
+See WTForms' documentation on internationalisation: https://wtforms.readthedocs.io/en/main/i18n/.

doc/index.rst

@@ -47,3 +47,9 @@ Indices And Tables
 * :ref:`genindex`
 * :ref:`modindex`
 * :ref:`search`
+
+
+.. toctree::
+   :hidden:
+
+   api/i18n