docs: edit docs

Author: ruanyf

docs/cli-options.md

@@ -34,6 +34,10 @@ $ loppo --site "My Documents"
 $ loppo --theme oceandeep
 ```
 
+##  --id
+
+`--id` sets a site's ID (default is the dir name of the project).
+
 ## --direction
 
 `--direction` sets the document's character direction. `ltr` is the default. It also could be setted as `rtl`.

docs/template-variables.md

@@ -9,6 +9,7 @@ They could be divided into two categories: site variables and page variables.
 The site variables are the same within the whole site.
 
 - site
+- id
 - dir
 - chapters
 - chaptersOrigin
@@ -17,7 +18,11 @@ The site variables are the same within the whole site.
 
 ### option.site
 
-`option.site` is the site name.
+`option.site` is the site name (default is `Documents`).
+
+### option.id
+
+`option.id` is the site id (default is the dir name of the project).
 
 ### option.dir