Allow custom output path for `static` and `external_static` instead of only `static` (#884)

src/Writing/HtmlWriter.php

@@ -30,7 +30,7 @@ class HtmlWriter
         // If they're using the default static path,
         // then use '../docs/{asset}', so assets can work via Laravel app or via index.html
         $this->assetPathPrefix = '../docs/';
-        if ($this->config->get('type') == 'static'
+        if (in_array($this->config->get('type'), ['static', 'external_static'])
             && rtrim($this->config->get('static.output_path', ''), '/') != 'public/docs'
         ) {
             $this->assetPathPrefix = './';