Keep verification screenshots out of the repository

Browser checks leave screenshots behind, and they had been accumulating in the
same directory as the Försäkringskassan captures, which are reference material
for the scraper and belong in the repository. The two are now separated by
where they sit rather than by what they happen to be called: verification
output goes in an ignored subdirectory, and the note saying so sits next to
the layout it describes.

.gitignore

@@ -6,6 +6,10 @@ __pycache__/
 graphify-out
 backend/debug
 
+# Ephemeral browser-verification screenshots (the tracked captures/ files are
+# scraper reference material and stay tracked).
+/captures/verification/
+
 # C extensions
 *.so