Ensure html/js dir exists

Debug attempt for #1838

We write search.js to html/js, but ever since assets were
moved into being generated by an app[^1], this dir is gone
from the repo, which causes `make html` fail to build search file.

[1] f856a13ec12aa2abe0918

Author: zoffixznet

html/js/DUMMY.txt

@@ -0,0 +1 @@
+dummy file to keep dir alive