Go to core
database, then in content editor go to /sitecore/content/Applications/Content Editor/Editors
item. Create new folder item here if you like and inside create new item with this template: /sitecore/templates/Sitecore Client/Content editor/Editor
(you can copy any existing editor item into your folder). You can set header, icon and Url (any url, absolute e.g. http://stackoverflow.com or relative e.g. /sitecore/shell/Applications/Media/Media Folder.aspx
).
Now go to master
database and set this editor in Editors
field (inside template or specific content item)
I think you should move your Page.aspx
inside sitecore project (in layout folder or custom one)
then url should be /layouts/Page.aspx