docs: add logo to docs site nav bar

Signed-off-by: oppiliappan <[e-mail]>

cmd/dolly/main.go

···
 		os.Exit(1)
 	}
 
-	if !isValidHexColor(fillColor) {
+	if fillColor != "currentColor" && !isValidHexColor(fillColor) {
 		fmt.Fprintf(os.Stderr, "Invalid color format: %s. Use hex format like #FF5733\n", fillColor)
 		os.Exit(1)
 	}

docs/logo.html

···
+<div class="flex items-center gap-2 w-fit mx-auto">
+  <span class="w-16 h-16 [&>svg]:w-full [&>svg]:h-full text-black dark:text-white">
+    ${ dolly.svg() }
+  </span>
+  <span class="font-bold text-4xl not-italic text-black dark:text-white">tangled</span>
+</div>

docs/template.html

···
               ${ x.svg() }
               $if(toc-title)$$toc-title$$else$Table of Contents$endif$
             </button>
+            ${ logo.html() }
             ${ search.html() }
             ${ table-of-contents:toc.html() }
           </div>
···
         class="hidden md:flex md:flex-col gap-4 fixed left-0 top-0 w-80 h-screen
                bg-gray-50 dark:bg-gray-800 border-r border-gray-200 dark:border-gray-700
                p-4 z-50 overflow-y-auto">
+        ${ logo.html() }
         ${ search.html() }
         <div class="flex-1">
           $if(toc-title)$

nix/pkgs/docs.nix

···
   inter-fonts-src,
   ibm-plex-mono-src,
   lucide-src,
+  dolly,
   src,
 }:
 runCommandLocal "docs" {} ''
···
 
   # icons
   cp -rf ${lucide-src}/*.svg working/
+
+  # logo
+  ${dolly}/bin/dolly -output working/dolly.svg -color currentColor
 
   # content - chunked
   ${pandoc}/bin/pandoc ${src}/docs/DOCS.md \