Docs: Update to jekyll-theme-amethyst 2.8.1

https://github.com/qunitjs/jekyll-theme-amethyst/commits/v2.8.1/

* Add scroll-based highlighting of "current" heading to the TOC.
* Expand at most one sidebar group for the current page.

Author: Krinkle

docs/Gemfile

@@ -2,4 +2,4 @@ source "https://rubygems.org"
 ruby RUBY_VERSION
 
 # To apply changes, run `bundle update`.
-gem "jekyll-theme-amethyst", "2.7.2", group: :jekyll_plugins
+gem "jekyll-theme-amethyst", "2.8.1", group: :jekyll_plugins

docs/api/QUnit/module.md

@@ -220,7 +220,7 @@ QUnit.module('Database connection', {
 
 ### Hooks on nested modules
 
-_Back to [§ Hook order](#hook-order)._
+_See also [§ Hook order](#hook-order)._
 
 ```js
 QUnit.module('My Group', hooks => {