Include markdown files only

[ariciputi]

As I was writing in #55 this is my suggestion about how to avoid including any file laying around in the repo root dir when creating the site output directory.

While I was working on it, I've discovered that MkDocs already has an utility function which defines what a markdown file is. So I've decided to retain that definition and select the files in the root dir based on what MkDocs does.

While doing that I've also removed some unused import statements. Moreover I've also added an explicit max line length for black in the pyproject.toml.

Any feedback or concern are more than welcome.