Ignore mermaid.min.js

It is very long and tends to match a lot of search queries. It's not useful to show with ripgrep.

Note that this does not actually untrack the file; changes can still be committed (although I suspect it may not have been intentional to commit originally?). This just changes how it interacts with tools that use `.gitignore` as a default filter.

.gitignore

@@ -83,6 +83,7 @@ __pycache__/
 node_modules
 package-lock.json
 package.json
+/src/doc/rustc-dev-guide/mermaid.min.js
 
 ## Rustdoc GUI tests
 tests/rustdoc-gui/src/**.lock