feat(ci): add markdownlint-cli2 for consistent markdown formatting

Add automated markdown linting to ensure consistent formatting across
all markdown files in the repository.

Changes:
- Add .markdownlint-cli2.jsonc configuration
- Create .github/workflows/lint.yml for CI/CD integration
- Fix all 908 existing markdown errors across 27 files
- Enforce ATX-style headings and asterisk emphasis
- Set consistent 2-space list indentation

This prevents markdown errors after project initialization and
maintains high documentation quality standards.

.markdownlint-cli2.jsonc

@@ -0,0 +1,24 @@
+{
+  // https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md
+  "config": {
+    "default": true,
+    "MD003": {
+      "style": "atx"
+    },
+    "MD007": {
+      "indent": 2
+    },
+    "MD013": false,
+    "MD024": {
+      "siblings_only": true
+    },
+    "MD033": false,
+    "MD041": false,
+    "MD049": {
+      "style": "asterisk"
+    },
+    "MD050": {
+      "style": "asterisk"
+    }
+  }
+}