Add DOCTYPE preamble to Markdeep documents

@korigamik points out that we get a quirks mode warning in browsers when rendering our Markdeep documents, as they lack the `<!DOCTYPE html>` preamble. I'm in an email conversation with Morgan McGuire (Markdeep) about this, but can't think of a good reason _not_ to include this in our Markdeep documents. I believe we should add them to all HTML/Markdeep documents in the project.