Documentation: Extend html-formatter and CustomRendering

**Documentation request:**

Please add some examples on how to implement our own html-formatter based on the existing one.
It is difficult to understand how to properly use the "CustomRendering" to change the way different parts are rendered.

**Describe the solution you'd like**
A complete step-by-step guide that explains how to extend the html-formatter and how to override different elements' rendering.

Thank you for your help.