named_with_underscores.html (not web style dashes).generic.View) from django.forms.generic inherit from ContextMixinContextMixin defines the method get_context_data:
def get_context_data(self, **kwargs):
    kwargs.setdefault('view', self)
    if self.extra_context is not None:
        kwargs.update(self.extra_context)
    return kwargs
So when overriding one must be careful to extends super's kwargs:
def get_context_data(self, **kwargs):
    kwargs = super().get_context_data(**kwargs)
    kwargs['page_title'] = "Documentation"
    return kwargs