Add filter documentation in User Guide

docs/en/docs/syntax/filters.md

@@ -0,0 +1,37 @@
+# Filters
+
+Filters are used to format the rendered contents.
+
+Example:
+
+```ejs
+message: <%= "foo\nbar" | dbg %>
+```
+
+Output:
+
+```html
+message: &quot;foo\nbar&quot;
+```
+
+!!! Note
+    Since `dbg` filter accepts '<T: std::fmt::Debug>' types, that type isn't required to implement [`Render`](https://docs.rs/sailfish/latest/sailfish/runtime/trait.Render.html) trait. That means you can pass the type which doen't implement `Render` trait.
+
+
+## Syntax
+
+- Apply filter and HTML escaping
+
+```ejs
+<%= expression | filter %>
+```
+
+- Apply filter only
+
+```ejs
+<%- expression | filter %>
+```
+
+## Built-In Filters
+
+Built-In filters can be found in [`sailfish::runtime::filter`](https://docs.rs/sailfish/latest/sailfish/runtime/filter/index.html) module.

docs/en/docs/syntax/overview.md

@@ -32,6 +32,15 @@
 
 Unlike EJS, you cannot omit the file extension.
 
-## Helpers
+## Filters
 
-(Work in progress)
+```ejs
+<%= message | upper %>
+```
+
+```ejs
+{
+    "id": <%= id %>
+    "comment": <%- comment | dbg %>
+}
+```

docs/en/mkdocs.yml

@@ -50,3 +50,4 @@ nav:
     - 'Overview': 'syntax/overview.md'
     - 'Tags': 'syntax/tags.md'
     - 'Includes': 'syntax/includes.md'
+    - 'Filters': 'syntax/filters.md'