preview variables (#201)

* Update preview.md

* Update preview.md

* preview variables

---------

Co-authored-by: Andy Hsu <[email protected]>

Author: balh55y

docs/config/preview.md

@@ -54,8 +54,14 @@ the first key `extensions` is the file extensions that split by `,`(if it's star
 
 For the external url, we provide some variables for you to use:
 
+Basic variable:
+
 - `$url`: the file url
+- `$durl`: the file of alist URL
 - `$name`: the file name
+
+Extended variables:
+Add [eb_] before the basic variable, where e means `encodeURIComponent`, b means `base64`, such as
 - `$e_url`: encodeURIComponent($url)
 - `$b_url`: btoa($url)
 - `$eb_url`: encodeURIComponent(btoa($url))

docs/zh/config/preview.md

@@ -54,8 +54,14 @@ interface {
 
 对于外部 url，我们提供了一些变量供您使用：
 
+基础变量：
+
 - `$url`: the file url
+- `$durl`: the file of alist URL
 - `$name`: the file name
+
+拓展变量：
+在基础变量前添加[eb_]，其中e表示`encodeURIComponent`，b表示`base64`， 如
 - `$e_url`: encodeURIComponent($url)
 - `$b_url`: btoa($url)
 - `$eb_url`: encodeURIComponent(btoa($url))