queries: Inject markdown into Rust doc comments

Author: the-mikedavis

runtime/queries/rust/injections.scm

@@ -1,6 +1,10 @@
-([(line_comment) (block_comment)] @injection.content
+([(line_comment !doc) (block_comment !doc)] @injection.content
  (#set! injection.language "comment"))
 
+((doc_comment) @injection.content
+ (#set! injection.language "markdown")
+ (#set! injection.combined))
+
 ((macro_invocation
    macro:
      [