refactor: replace gulp by webpack and npm scripts (#258)

BREAKING CHANGE: We have replaced `gulp` with `webpack` and `npm scripts` to build this theme. If you build it on your own or use build commands during the deployment, you may have to adjust your setup.

BREAKING CHANGE: The `GeekblogIcons` font is using the icon name as Unicode now. As a consequence, you have to replace all references to Icons from this font if you have customized the theme.

BREAKING CHANGE: We have refactored the search integration to split Hugo templates from JavaScript code. To get it working again, you need to adjust the `outputFormats` and `outputs` in your Hugo configuration file, as [documented](https://geekdocs.de/usage/configuration/#site-configuration).

layouts/_default/_markup/render-heading.html

@@ -1,5 +1,7 @@
 {{- $showAnchor := (and (default true .Page.Params.GeekdocAnchor) (default true .Page.Site.Params.GeekdocAnchor)) -}}
 
+
+<!-- prettier-ignore-start -->
 {{- if $showAnchor -}}
 <div class="gdoc-page__anchorwrap">
     <h{{ .Level }} id="{{ .Anchor | safeURL }}">
@@ -16,3 +18,4 @@
     </h{{ .Level }}>
 </div>
 {{- end -}}
+<!-- prettier-ignore-end -->