Give each template its own README

The root README keeps what is shared — dependencies, accessibility, the
scripts — and links to a per-template README for how to build and what is
in it. The Posters section moves to poster_template/README.md with a
pointer left behind, since it is that template's caveat.

slides_template/README.md replaces the two READMEs that came with
beamer-mako.

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

poster_template/README.md

@@ -0,0 +1,20 @@
+# Poster Template
+
+A conference poster on beamer with beamerposter, styled by the Torino theme
+and `beamercolorthememako.sty`. The source file is `main.tex`.
+
+```
+latexmk -f -lualatex main.tex
+```
+
+## Not accessible
+
+This is the one template that produces an untagged PDF, and it makes no
+accessibility claim. The beamer class rejects `\DocumentMetadata`, and the
+LaTeX Tagging Project lists beamer and beamerposter as
+[no-support](https://latex3.github.io/tagging-project/tagging-status/), meaning
+incompatible and not expected to change.
+
+When a poster needs an accessible version, build a companion document from the
+[paper](../paper_template/README.md) or [memo](../memo_template/README.md)
+template.