We no longer use the `--root` option of `build_website.py` since we have
moved to jqlang.org (from jqlang.github.io/jq where the root was `/jq`).
We don't need the option on local development either since `http.server`
has the `-d` option to specify the directory to serve from.
* Bump up Bootstrap to v5.3.1, Bootstrap Icon to v1.10.5.
* Use autoComplete.js to drop dependency on jQuery and typeahead.js.
* Support dark mode.
* New svg logo and icon with responsive color mode support.
* Normalize section ids to lower kebab-case for easiness of linking.
* Use relative paths for links for local development (--root /output).
* Various markup cleanups and accessibility improvements.
