doc (cli/blog): Document new --alt-link option

doc/libervia-cli/blog.rst

@@ -23,6 +23,21 @@ The `set` and `edit` commands support a range of options for customizing blog po
 
       Use this option judiciously to avoid unintended overwriting of blog posts.
 
+``--alt-link URL [MEDIA_TYPE]``
+
+   Add an alternative link to a blog post, such as a corresponding HTTP page. The *URL*
+   parameter is mandatory, while *MEDIA_TYPE* is optional. Incorporate ``{service}``,
+   ``{node}``, and ``{item}`` as template values within the URL. These will be replaced
+   with the actual values when the item is published. To use curly braces literally,
+   double them: ``{{`` and ``}}``.
+
+example
+-------
+
+Louise  wants to add a link to her public facing HTML blog in her item::
+
+  $ li blog set --alt-link https://example.org/blog/{node}/{item}
+
 set
 ===