Reverted previous default assignment of compiler.rootRelativeImageURL.

This broke the case where config.rootRelativeImageURL is set to
true --> empty string --> false, which is treated differently by the compiler.

Added descriptin in configuration.md

Author: weijunyu

docs/configuration.md

@@ -616,3 +616,24 @@ window.$docsify = {
   topMargin: 90, // default: 0
 };
 ```
+
+## rootRelativeImageURL
+
+- Type: `Boolean|String`
+- Default: `false`
+
+Configure image URL rendering behavior when inserting markdown images when the image URL is root-relative, i.e. starts with '/'.
+
+By default, Docsify resolves all image paths against the current page's parent path.
+
+E.g. if the current path is `/advanced/guide`, `![](/assets/image.png)` would render as `<img src="advanced/assets/image.png">`.
+
+```js
+  window.$docsify = {
+    rootRelativeImageURL = false // default behaviour
+
+    rootRelativeImageURL = true // ![](/assets/image.png) renders as <img src="/assets/image.png" />
+
+    rootRelativeImageURL = 'my-root-path' // ![](/assets/image.png) renders as <img src="/my-root-path/assets/image.png />
+  }
+```

src/core/render/compiler.js

@@ -68,7 +68,7 @@ export class Compiler {
     this.linkRel =
       this.linkTarget === '_blank' ? config.externalLinkRel || 'noopener' : '';
     this.contentBase = router.getBasePath();
-    this.rootRelativeImageURL = config.rootRelativeImageURL || false;
+    this.rootRelativeImageURL = config.rootRelativeImageURL;
 
     const renderer = this._initRenderer();
     this.heading = renderer.heading;