[feature request]: Tutorial source links to be functional in markdown

[bigtimebuddy]

Related issue: https://github.com/pixijs/guides/issues/15

Problem is that markdown file source links (e.g., [Scene Graph](scene-graph.html)) need to be expressed for the output file name. These links don't work when the tutorials are in markdown format (e.g., [Scene Graph](scene-graph.md)). Would be great to convert markdown file links to HTML file links when building.