Remove hard connection between SITEURL and 'Blog' in navbar

This small change removes the "Blog" -> `SITEURL` entry in the navigation. The `MENUITEMS` setting now fully controls the contents of the navbar, which is important if the user is defining a custom landing page for a site and doesn't want to identify the `SITEURL` as `Blog`, or if they want to call it something else (e.g. `NOTES`). This is the general convention used in the Pelican default theme as well.