chore(config): Add markdownlint configuration file

Adds a configuration file for markdownlint with custom rule settings.
The configuration enables all rules by default with specific adjustments
for list indentation, line length, duplicate headings, and more.

.markdownlint.yaml

@@ -0,0 +1,21 @@
+---
+# Default to enable all rules
+default: true
+
+# MD007: unordered-list-indent
+MD007:
+  indent: 4
+
+# MD013: line-length
+MD013: true
+
+# MD024: no-duplicate-heading
+MD024:
+  siblings_only: true
+
+# MD033: no-inline-html
+MD033: false
+
+# MD041: first-line-h1
+MD041: false
+...