Move html page structure to separate file

[kordwarshuis]

The final templating step is now inside the main Node.js file (/index.js).

This template html should be moved to a separate file.

• /index.js is more readable;
• creates the possibility for a templating system
• less error-prone

[kordwarshuis]

Moved html to /templates/template.html

Possible future steps: create templating systems -> create option to add classes to TOC (table of content) elements (<ul> and <li>)