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).

assets/search/searchdata.json

@@ -0,0 +1,12 @@
+[
+  {{ range $index, $page := (where .Site.Pages "Params.GeekdocProtected" "ne" true) }}
+    {{ if ne $index 0 }},{{ end }}
+    {
+      "id": {{ $index }},
+      "href": "{{ $page.RelPermalink }}",
+      "title": {{ (partial "title" $page) | jsonify }},
+      "parent": {{ with $page.Parent }}{{ (partial "title" .) | jsonify }}{{ else }}""{{ end }},
+      "content": {{ $page.Plain | jsonify }}
+    }
+  {{ end }}
+]