You can add it in the form of Static site pages. These can also be external links. 

For adding a site page go to Manage >content>  click on pages

And click on add new page.

You can add to types of Site pages.

a) Page Content - This is the page on which you can write your own content.

b) Redirect to a link - In this case, you can provide an external link of the page.

and click to save and upload.