tpl: Add sha256 template function

Fixes #2742

content/templates/functions.md

@@ -768,6 +768,17 @@ This can be useful if you want to use Gravatar for generating a unique avatar:
 <!-- returns the string "c8b5b0e33d408246e30f53e32b8f7627a7a649d4" -->
 ```
 
+
+### sha256
+
+`sha256` hashes the given input and returns its SHA256 checksum.
+
+```html
+{{ sha256 "Hello world, gophers!" }}
+<!-- returns the string "6ec43b78da9669f50e4e422575c54bf87536954ccd58280219c393f2ce352b46" -->
+```
+
+
 ## Internationalization
 
 ### i18n