Add Next and Back Navigation Buttons to the Documentation

[corwinjoy]

Add navigation buttons to the documentation as per the proposal here: https://github.com/duckdb/duckdb-web/issues/2852

Comments:

1. I was able to get this to work pretty well but there were a couple limitations. a. The Sitemap appears twice, once at the top level and once at the beginning of the documentation. I had to pick one to get a single navigation order so I chose the Sitemap position in the left menu. b. I did not include navigation for the demo page since I think that makes no sense.

2. I made the buttons as pretty as I know how, but I'm not all that great at graphic design. The buttons are a class in the css file so I welcome suggestions there.

3. It might make sense to have navigation buttons at the top of the page as well, but I am not as sure how, or if, I should do that.

Thanks in advance for taking a look! I do think this does help to improve the docs.