Put the crosspoint screenshot in the user guide, and ship a PDF

The guide opens by explaining the crosspoint model in prose to somebody
who, if they have used an SDI router, already recognises it on sight. The
screenshot was sitting in docs/screenshots unused.

docs/USER-GUIDE.pdf is generated from this file by the website's
scripts/build_guides.py — edit the markdown, never the PDF. It was
generated when the guide pipeline was first built but only landed in the
website repo; this adds the repo's own copy.

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

Author: stoatworks-labs

docs/USER-GUIDE.md

@@ -21,6 +21,11 @@ the other, where each destination is fed by exactly one source, switchable live.
 
 Switching an output from one source to another is the whole point, and it happens live.
 
+![The crosspoint grid in the web UI: sources across the top, outputs down the side, and a marked cell where an output is taking a source.](screenshots/crosspoint-grid.png)
+
+*One marked cell per row, because an output takes exactly one source. A column with several marks
+is one source feeding several outputs, which is allowed and normal.*
+
 ---
 
 ## Running it