feat: auto-load custom svg sprites from assets folder (#79)

BREAKING CHANGE: Build-in icons are prefixed with `gdoc_` now. If you use build-in icons in e.g. menus you have to rename these references!

exampleSite/data/menu/more.yaml

@@ -2,12 +2,12 @@
 more:
   - name: News
     ref: "/posts"
-    icon: "notification"
+    icon: "gdoc_notification"
   - name: Releases
     ref: "https://github.com/thegeeklab/hugo-geekdoc/releases"
     external: true
-    icon: "download"
+    icon: "gdoc_download"
   - name: "View Source"
     ref: "https://github.com/thegeeklab/hugo-geekdoc"
     external: true
-    icon: "github"
+    icon: "gdoc_github"