Describe in the docs how to add a PDF download button

[timvink]

Basically how to manually add a non-markdown file to the MkDocs site.

I envision the user pointing to the location of the PDF file in the config, and the plugin then taking care of adding the file and a page.pdf_url parameter to every page. Then you can overwrite a page content partial.

We could use the fontawesome icon file-pdf

<i class="fas fa-file-pdf"></i>

We add an extra config param pdf_url, with the relative url from the docs/ dir. All files in docs are copied over (link).

Just need to figure out the correct link to the file. Perhaps for each page find the os.path.dirname() of the url_relative_to() the index page. Maybe there's a variable with the homepage.