[Issue 12536] Improve the documentation about preprocessing the files has "Ddoc" file header

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12536
Issue 12536 depends on issue 12535, which changed state.

Issue 12535 Summary: The language introduction page is not linked from index
https://issues.dlang.org/show_bug.cgi?id=12535

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--