You could hard code the navigation, but the problem is that if you add new pages (or change their names), you'd have to go through the trouble of updating it in your code.

I have a FREE theme that you're welcome to use as a starting point.
http://www.concrete5.org/marketplace/themes/profolio...

You could hard code anything you want if you so desired. You could have the entire website inside concrete5 with nothing actually being editable on the site....

But instead of hard coding the navigation, I suggest making a new global area and dropping an auto-nav block in its place. The styling markup is the same for any ul > li menu, I like to wrap my auto nav area in a <nav> tag.

The included header file just makes it easy for updating the header if you have multiple page types. So you could just have default.php and nothing more, but if you have default.php, then blog.php, and so on, it'd be wise to include the header from another file incase you want to update that part without changing every page template file.

Another good starter theme for building custom templates is cloneamental:
http://www.concrete5.org/marketplace/themes/cloneamental...