15849: Adds symlink to script from doc's includes subdir.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <ldipentima@veritasgenetics.com>

diff --git a/doc/_includes/_vocabulary_migrate_py.liquid b/doc/_includes/_vocabulary_migrate_py.liquid
new file mode 120000 (symlink)
index 0000000..9cd3629
--- /dev/null
+++ b/doc/_includes/_vocabulary_migrate_py.liquid
@@ -0,0 +1 @@
+../../tools/vocabulary-migrate/vocabulary-migrate.py
\ No newline at end of file