Use plain Mermaid diagrams again and drop the SVG images

The preview problems were caused by the diagram text, which is now fixed in
every copy, so the pre-rendered SVGs (white background, harder to keep in
sync) are no longer needed. Also updates docker/docs with the fixed text.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -12,8 +12,8 @@ X25519 key agreement so the broker password is never transmitted.
     broker username, with the threat model.
   - [docs/enrollment-and-activation.md](docs/enrollment-and-activation.md): the FN-as-username and SN-as-username models compared.
   - [docs/mqtt-dynamic-security-rnd.md](docs/mqtt-dynamic-security-rnd.md): the original R&D notes.
-  - The diagrams are written and checked against **Mermaid 11.13.0** (the version bundled with Typora and Obsidian);
-    the sequence diagrams are also embedded as SVG images rendered with that version, with the source under each.
+  - The diagrams are plain Mermaid. They avoid `;`, `|`, `+`, `{}` and `<...>` inside sequence-diagram messages, which
+    broke the GoLand and Typora previews, and parse on Mermaid 11.13.0 (the lowest version those tools bundle).
 - `go/`, `c/`, `c-libsodium/`, `cpp/`, `run.sh`, `run-libsodium.sh`: fixed-key programs that check the
   crypto libraries against the RFC test vectors and against each other.