Fix svg height in mermaid; mermaid version

docs/style.css

@@ -1,3 +1,7 @@
 .mermaid {
   max-width: 350px;
 }
+
+.mermaid svg {
+  height: auto;
+}

mkdocs.yml

@@ -31,7 +31,7 @@ plugins:
     - search
 
 extra_javascript:
-    - https://unpkg.com/mermaid/dist/mermaid.min.js
+    - https://unpkg.com/mermaid@8.7.0/dist/mermaid.min.js
     - mermaid.js
 
 extra_css: