Issue 12535: Fix link to the Introduction page

Fix: https://issues.dlang.org/show_bug.cgi?id=12535

This request is to revive the Introduction page missed in D2.
I doubt it is intended that the page has not been linked.
http://dlang.org/intro.html

Moreover, I would like to apped it a few.
See https://issues.dlang.org/show_bug.cgi?id=12536