Document how to do frontend Bootstrap styling in customization docs

In `docs/customization.qmd`:

- [ ] Document how to add some custom theming with Bootstrap SASS
- [ ] Document how to replace bootstrap with a different styling system?

We'll need to download Bootstrap source files:

https://getbootstrap.com/docs/5.3/getting-started/download/#source-files

Then follow the customization instructions here:

https://getbootstrap.com/docs/5.0/customize/sass/