feat: doc pages can have icons shown before the title

assets/_main.scss

@@ -287,6 +287,12 @@ body[dir="rtl"] .book-menu {
   will-change: transform, margin, opacity;
 }
 
+.book-filetree-icon {
+  width: 16px;
+  height: 16px;
+  vertical-align: middle;
+}
+
 @media screen and (max-width: $mobile-breakpoint) {
   #menu-control,
   #toc-control {

layouts/partials/docs/title.html

@@ -3,7 +3,11 @@
   Accepts Page as context
 -->
 {{ $title := "" }}
+{{ $icon := "" }}
 
+{{ if .Params.Icon }}
+  {{ $icon = printf "<img src=\"%s\" class=\"book-filetree-icon\" />" .Params.Icon }}
+{{ end }}
 {{ if .LinkTitle }}
   {{ $title = .LinkTitle }}
 {{ else if .Title }}
@@ -14,4 +18,5 @@
   {{ $title = .File.BaseFileName | humanize | title }}
 {{ end }}
 
-{{ return $title }}
+{{ $out := printf "%s %s" $icon $title | safeHTML }}
+{{ return $out }}