Add examples for `bundleExportKey` feature to docs

bundleExportKey: "bundle" (this is the default value) means that you can do this in your 11ty.js template:

export const bundle = {
	css: "/* Only adds to css bundle */"
};

export function render() {};
export function data() {};

This works with buckets too, if you nest another object in your export:

export const bundle = {
  css: {
    default: "/* Only adds to default bucket in css bundle */"
  }
};

This works too but probably isn’t what you want:

export const bundle = "/* Adds to all bundles using bundleExportKey: "bundle" */";

This makes more sense, if you want to opt-in to something a bit simpler with a string export direct to a bundle:

bundleExportKey: "css" means that you can do this in your 11ty.js template:

export const css = "/* Some CSS */";

Start by locating the documentation entry for bundleExportKey and review the 11ty.js examples described in the issue. Add examples for the default bundle key, nested buckets, direct string exports, and a custom css key; done means the supported configurations and their effects are clear in the docs.