When using MarkdownRenderer allow setting Title

[s-martin]

Is your feature request related to a problem? Please describe.

While I can set that the title shall be rendered with render_page_title, I can't set the title itself. It always processes to None. It's also not consistent to the behavoir of render_toc and render_toc_title.

Describe the solution you'd like

I'd like to set the title.