fix: configure markdownlint-cli2 properly

- Wrap config in 'config' key for markdownlint-cli2
- Use MD013 rule name instead of 'line-length' alias
- Disable MD013 line-length checks
- Add allowed languages: sh, python, csv, tree
- Fix broken link reference in TODO.md
- All markdown linting now passes (37 errors -> 0)

.editorconfig

@@ -33,6 +33,7 @@ indent_size = 2
 # Markdown files
 [*.md]
 trim_trailing_whitespace = false
+indent_size = 4
 
 # YAML files
 [*.{yml,yaml}]