add banner for unstable documentation

Author: tapaswenipathak

readthedocs/projects/migrations/0024_add-show-version-warning.py

@@ -15,7 +15,7 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='project',
             name='show_version_warning',
-            field=models.BooleanField(default=False, help_text='Show warning banner in non-stable nor latest versions.', verbose_name='Show version warning'),
+            field=models.URLField(blank=True, help_text='You are seeing unstable version of the documentation, see the stable latest versions.', verbose_name='Show version warning'),
         ),
         migrations.AlterField(
             model_name='domain',

readthedocs/projects/models.py

@@ -248,10 +248,10 @@ class Project(models.Model):
         default=False,
         help_text='If checked, do not show advertising for this project',
     )
-    show_version_warning = models.BooleanField(
+    show_version_warning = models.URLField(
         _('Show version warning'),
-        default=False,
-        help_text=_('Show warning banner in non-stable nor stable latest versions.'),
+        blank=True,
+        help_text=_('You are seeing unstable version of the documentation, see the stable latest versions.'),
     )
 
     # Sphinx specific build options.