Dashboard editing and understanding mechanics for this

Hi everyone!
I have a problem…
I want add some changes to dashboard page like add new link in Discover block.
I tried to search some information about dashboard or core mechanics (how it works) and find only page
But when I tried to open ‘sitemesh’ page, I got 404 error…and it was happened. with all links bellow.
Maybe. somebody can describe how edit dashboard simply?