util/markdownlint.rb: Add two rule exceptions: MD023 and MD026

exclude_rule 'MD023' # Headers must start at the beginning of the line
exclude_rule 'MD026' # Trailing punctuation in header

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12109)

util/markdownlint.rb

@@ -15,6 +15,8 @@ exclude_rule 'MD004' # Unordered list style TODO(fix?)
 exclude_rule 'MD005' # Inconsistent indentation for list items at the same level
 exclude_rule 'MD006' # Consider starting bulleted lists at the beginning of the line
 exclude_rule 'MD014' # Dollar signs used before commands without showing output
+exclude_rule 'MD023' # Headers must start at the beginning of the line
 exclude_rule 'MD024' # Multiple headers with the same content
 exclude_rule 'MD025' # Multiple top level headers in the same document
+exclude_rule 'MD026' # Trailing punctuation in header
 exclude_rule 'MD029' # Ordered list item prefix