Website mostly jekyll-ified.

Pages are now defined in terms of layouts (`_layouts`) and html code that is injected, from files in `_includes` and from html defined in the directory. Not sure this is the most convenient way, but at least the code repetition is minimal now.

The news page needs a layout (in _layouts/news.html) and must be filled.

The miscellaneous menu needs to be fixed.

Texts have not been modified/added.