fix(ci): fix markdownlint config for super-linter

- Move MD007/MD030 config to .markdown-lint.yml (the default file
  that MARKDOWN_CONFIG_FILE points to in super-linter v8)
- Remove .markdownlint.json (was never loaded, also violated
  EDITORCONFIG tab requirement for JSON files)

Author: dgibbs64

.github/linters/.markdown-lint.yml

@@ -9,6 +9,15 @@ MD051: false
 # MD013: The project documentation frequently contains long URLs/commands.
 MD013: false
 
+# MD007: Repo uses 2-space indentation for nested lists.
+MD007:
+  indent: 2
+
+# MD030: Repo uses 1 space after list markers (single and multi-paragraph).
+MD030:
+  ul_single: 1
+  ul_multi: 1
+
 # MD033: The main README uses inline HTML for badges.
 MD033: false