Reorganized operators under documentation for #20

* We now have a documentations submenu on the top nav bar and on
the footer of every page.
* `documentation/operators.html` is a list of all operators, linking
to their individual pages
* `documentation/operators/buffer.html` is the canonical example of
how an operator page should be layouted. The collape behavior was
jquery but it was replaced with bootstrap's own collapse. Check
buffer.html to see the usage of `{% lang_operator %}`, a custom Jekyll
tag.