docs: de-dup README/CHANGELOG (keep .adoc, port sponsor badge) (#23)

Family-wide doc de-dup that landed across the -iser repos during base
camp but was missed here (this repo was squash-merged before the
cleanup).

- Remove stale `README.md` (`README.adoc` is canonical), porting the
GitHub sponsor badge into `README.adoc`.
- Remove duplicate `CHANGELOG.md` (`CHANGELOG.adoc` is canonical).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01Mbq6yKF9RhFai6EQ7WqKhQ

---
_Generated by [Claude
Code](https://claude.ai/code/session_01Mbq6yKF9RhFai6EQ7WqKhQ)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

CHANGELOG.md

@@ -7,6 +7,8 @@ Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 :icons: font
 :source-highlighter: rouge
 
+image:https://img.shields.io/badge/Sponsor-%E2%9D%A4-pink?logo=github[Sponsor,link=https://github.com/sponsors/hyperpolymath]
+
 == What Is a2mliser?
 
 a2mliser wraps any markup, configuration, or manifest file in an