Add Markdown linting

Add linting for Markdown files, following norms from other repos.

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

.markdownlint.yml

@@ -0,0 +1,12 @@
+---
+# Breaks reusing MD snippets extracted to files
+first-line-heading: false
+
+# Set maximum line Length to 140c to match Go linting
+line-length:
+  line_length: 140
+
+# Allow HTML span elements to set font sizes
+no-inline-html:
+  allowed_elements:
+    - span