In your custom app, add crm.py in config folder with the code for your link
from __future__ import unicode_literals
from frappe import _
def get_data():
return [
{
"label": _("Settings"),
"items": [
{
"type": "doctype",
"name": "Your doctype",
"label": "the text you want to display",
},
]}]
You can only add, not replace.