If you're looking insert renderable elements into the form array in the same manner as the grouped vertical tabs, you can hook_form_alter()
your way to this via adding a details
form element:
function MYMODULE_form_alter(&$form, $form_state, $form_id) {
// @TODO: Add your own logic for filtering relevant node forms
if (in_array($form_id, array('node_page_form', 'node_page_edit_form'))) {
$form['example'] = array(
'#type' => 'details',
'#title' => t(''),
'#description' => t(''),
'#group' => 'advanced',
'#open' => TRUE,
'markup' => array(
'#markup' => '<p>Random Markup</p>',
),
'#weight' => 1000,
);
}
}
If you're looking to add markup that's not relevant to your node forms, you might be better off subtheming Seven and adding your own node-edit-form.html.twig template to your subtheme with your own markup additions:
<div class="layout-node-form clearfix">
<div class="layout-region layout-region-node-main">
{{ form|without('advanced', 'actions') }}
</div>
<div class="layout-region layout-region-node-secondary">
{{ form.advanced }}
<p>Random Markup</p>
</div>
<div class="layout-region layout-region-node-footer">
{{ form.actions }}
</div>
</div>