Add render_file helper function to setup.rb

[#66423154]

It will be used to render embeddable, embeddable-dev
and embeddable-staging pages, which will be almost identical.
Add embeddable, embeddable-dev and embeddable-staging pages

[#66423154]

Old embeddable.html.haml is renamed to embeddable.haml and used
as a template for other pages.
Update Makefile, add new embeddable pages

[#66423154]

Also HAML_FILES is modified to include only .html.haml files, so we can
use .haml as a template that is not expected to be converted to HTML.