I want to keep stackexchange site as a notification so that whenever I want to open the site I just need to scroll down from the top and click on link of the site. Is there any way to do that?
No, there is no way I am aware of of simply adding arbitrary links that do not expire once viewed. You could create reminders or calendar links or any other sort of repeated notification which contain the URLs, but this is a most inelegant solution.
If quick access to the SE site is what you need, then you could put a link in your bookmarks menu or bar, or you could even place the sites in your reading list which means it's always in the top section after clicking bookmarks (assuming you don't use reading list for it's intended purposes.
If you want to open a URL without opening Safari first, the best option may be to go to the SE homepage, click the share icon and click
I would look at Launch Center Pro, an inexpensive shortcut app for the iPhone.
With it, you can set up a custom action for Safari (and many other apps) which will automatically open a link.
To create what you'd want, create a new action by first tapping the pencil icon in the upper right, then one of the plus buttons, then New Action and then either Bookmark or Safari > Bookmark.
To make it easily accessible, I have LCP on the dock of my iPhone.