delete robots.txt

This is no longer needed. It was used long ago to avoid robots
traversing `ocaml.org/master` where we were publishing a dev preview of
the site.
remove old obsolete build scripts
merge new and old README files
move all files from src/ one level up

As requested in #227.
remove old template in src/lib

Only content of src/lib was the previous template used with Weberizer.
rename tpl/ to template/

As discussed in #227.
rename app/ to script/

In anticipation of also putting bash scripts in here. The term `app`
implies something bigger than simple scripts.
.gitignore: remove src/lib/ items

Should have been done with 915be7ca2b9dd83b95fd66ca04b3f6fccfdea419.
move all script files to script/ directory