Use HTML comment marker for include start

Change the include start marker in MANUAL.md from the visible heading '## Installation' to an explicit HTML comment '<!-- Install start -->'. Add this comment to README.md just before the Installation section so the include can target a stable, non-visible marker instead of matching visible heading text.

Author: douden

MANUAL.md

@@ -70,7 +70,7 @@ This content will appear in the sticky margin when the original element scrolls
 ```
 
 ::::{include} README.md
-:start-after: "## Installation"
+:start-after: "<!-- Install start -->"
 ::::
 
 ## Additional content

README.md

@@ -4,6 +4,7 @@
 
 When the original element scrolls above the header, a duplicate appears in the right margin (on wide screens). When the original element comes back into view, the margin copy is hidden.
 
+<!-- Install start -->
 ## Installation
 
 ```bash