Responsible note under the title; followup pages out of the nav until a chain exists

- The "Asked by X" line moves from the page footer to directly below
  the hero, where the question it belongs to is.
- New Question.followup flag: a post-submission page only appears in
  the question nav once the visitor's context actually carries an
  answer chain - nobody qualifies for "what happens now" before
  something happened.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/content.rs

@@ -25,6 +25,11 @@ pub struct Question {
     /// on the page.
     #[serde(default)]
     pub responsible: Option<Responsible>,
+    /// A page that only makes sense after answering something (the
+    /// post-submission pages) - kept out of the question nav unless the
+    /// visitor's context carries an answer chain.
+    #[serde(default)]
+    pub followup: bool,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize)]