There's one HTML file in this directory and the others are in seagull/static/html/ because seagull is a single page application.
The index.html is used by Beego framework and has the common part of all these pages. Here's the entry of the website of seagull.
It uses <div ng-view></div>
to include other pages.
And we have to load the HTML files as resources, so we put them in seagull/static/html/.