Document what the accessibility support does and does not do

Slides-only builds validate as PDF/UA-2, which is what upload targets
check for, but only frame titles and links are actually tagged. Say so
rather than leaving the veraPDF pass to imply more than it means.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

slides_template/example.tex

@@ -153,6 +153,8 @@ BoldItalicFont  =   *-BoldItalic
 }
 
 % packages i use in essentially every document
+% give every \includegraphics an alt={...} description. the current
+% toolchain drops it, but it will be used once tagging is available
 \usepackage{graphicx}
 \usepackage{url}
 % \usepackage{dcolumn}