[documentation] Broken links to images in "Tree Structure" page

[izikeros]

Images are not displayed on "Tree Structure" page of documentation:
https://pmorissette.github.io/bt/tree.html

Broken elements:

<img alt="simple tree structure" class="align-center" src="_static/tree1.png">

<img alt="advanced tree structure" class="align-center" src="_static/tree2.png">

There seem to be no such files in the _static directory of documentation.