Mezzanine is a CMS and so yes, after logging into the admin area, among other things, you can edit page contents and add new pages (even though our page contents are synced from the `page_contents` directory in the repo; will document this later).
Do no touch any of the markdown to include HTML, please. In the `geany/templates` folder you find the HTML templates which are used to render the basic site and the markdown contents are rendered dynamically into the `main` block (see `geany/templates/skel.html`). Only the homepage itself(`geany/templates/home.html`) is plain HTML without any Markdown, so there you can use fancy Bootstrap cards and so on.