Add a logo to the navigation bar of the HTML rendered book

[scattenlaeufer]

I added the option to add an image as logo to the navigation bar by adding a configuration parameter to the book table of the book.toml. This resolves #877

I also thought about adding the option of a project logo to the bottom of the navigation sidebar, but wanted to wait for feedback to this first edit before adding more code.

[ehuss]

I'm wondering, would it be possible to make this part of the SUMMARY instead of making it a configuration option? That is, before the chapter list, allow an image to be specified. It could be constrained to only specific events for now (like images and maybe HTML?). That would alleviate the issues with needing to specify alt text, image width/height, and other details. I'm a little concerned about trying to specify things in TOML since specifying details like those can be difficult.

[montyly]

Hi. Is there any update on this PR? It's a neat addition