you would not run ERPNext on the same server as your website in order to avoid dependencies interferring with each other
displyaing your ERPNext instance under any URL (such as shahrmedia.com/erpnext) naturally is possible (I think even with a cloud instance running on erpnext.com i.e.). I’ll dig out a how-to for you and post is here, give me a second
I think the key here may be the following information at the very top of the howto you are consultiing
“… edited this page on Jun 24, 2013”
it is pretty old and i would be surprised whether it would still work. you can find manuals on how to install ERPNext here in this forum or i.e. here.
please be also advised that managing your own ERPNext instance which is exposed to the internet is somethign that requires quite a level of Linux system administration expertise if you want to have a safe and secure system. Many people may be better off with a hosted version of ERPNext like on erpnext.com.
This article may be an interesting read in this context
The title of your post mentions that you are using cPanel. This is not the way to host ERPNext. It may be possible to install from within cPanel but it would be a total nightmare. Typically you need to install ERPNext at a seperate sub domain like myerpnext.shahrmedia.com not shahrmedia.com/myerpnext Where myerpnext.shahrmedia.com points to a totally seperate server IP address. You could try to setup a digital ocean droplet or alternatively save your self a massive amount of hassle and host on ERPNext Cloud. Good Luck