Make sidebar width customizable

[dz4k]

Discussed in https://github.com/bigskysoftware/missing/discussions/37

^{Originally posted by t1 November 12, 2023} Missing is just cool. Thanks!

I need a little bit more space in my sidebar. I didn't find a hint in the docs and my experiments with using a grid actually flipped it to horizontal 🤔. Do I really have to resort to a width style attribute?

Whatever the solution, it might be worth adding a hint to the docs.