Allow HTML <a> element in MD, support heading IDs

The MD linter follows CommonMark, and in CommonMark the only way to add
custom ID tags to headings is by using HTML <a id="short-id"></a>.

Other syntaxes are valid in some MD syntaxes, but not CommonMark.

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

.markdownlint.yml

@@ -7,8 +7,10 @@ line-length:
   line_length: 140
 
 # Allow HTML span elements to set font sizes and for shortcodes
+# Allow <a id="short-heading"></a> to support [Link](#short-heading)
 no-inline-html:
   allowed_elements:
+    - a
     - code
     - div
     - li