Given a Sphinx builder and context with sphinx_app in it,
         generate HTML

def render(self, sphinx_app: Sphinx, context):
        """ Given a Sphinx builder and context with sphinx_app in it,
         generate HTML """

        # Called from kaybee.plugins.widgets.handlers.render_widgets

        builder: StandaloneHTMLBuilder = sphinx_app.builder
        resource = sphinx_app.env.resources[self.docname]
        context['sphinx_app'] = sphinx_app
        context['widget'] = self
        context['resource'] = resource

        # make_context is optionally implemented on the concrete class
        # for each widget
        self.make_context(context, sphinx_app)

        # NOTE: Can use builder.templates.render_string
        template = self.template + '.html'
        html = builder.templates.render(template, context)
        return html