ci: add index.html symlink to latest stable version's intro

Author: leovalais

.howdocgen

@@ -1,6 +1,8 @@
 #!/bin/bash
 set -x
 
+LATEST=$(find doc -maxdepth 1 -type d -not -name doc -not -name dev -exec basename {} \; | sort -nr | head -n 1)
+
 wget https://raw.githubusercontent.com/ocaml/ocaml-travisci-skeleton/master/.travis-ocaml.sh
 export OCAML_VERSION=4.06
 bash -ex .travis-ocaml.sh
@@ -32,5 +34,7 @@ cat >$f <<EOF
 EOF
 quickdop -f doc _doc -t json -c $f -viu
 
+ln -s "$LATEST/manual/intro.html" _doc/index.html
+
 # TODO Also remove that line.
 mv tmp _doc