Use MD046

Summary: https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md#md046---code-block-style

Differential Revision: D70258997

fbshipit-source-id: 7d4db5c57980474f35d94de32818fac36e0181e6

Author: dafyddcrosby

.mdlrc

@@ -1,6 +1,7 @@
 all
 rule 'MD003', :style => :setext_with_atx
 rule 'MD013', :code_blocks => false, :tables => false
+rule 'MD046', :style => :fenced
 
 # "Headers should be surrounded by blank lines" - we start our
 # sections directly below the header