Add a doctest for the `blank_line()` method

This adds a simple doctest for `blank_line()` but will not be included in the original PR because it is contingent on the merge of a pending PR for adding the `to_string()` method.

gemtext/src/lib.rs

@@ -23,6 +23,17 @@ impl Builder {
     /// This is equivilent to calling [`text()`] with an empty string, or pushing a blank
     /// [`Node`]
     ///
+    /// ```
+    /// # use gemtext::Builder;
+    /// let greeting = Builder::new()
+    ///     .text("Hello")
+    ///     .blank_line()
+    ///     .text("universe")
+    ///     .to_string();
+    ///
+    /// assert_eq!(greeting.trim(), "Hello\n\nuniverse");
+    /// ```
+    ///
     /// [`text()`]: Self::text()
     pub fn blank_line(mut self) -> Self {
         self.nodes.push(Node::blank());