Separate the content of the layout

[bjohansebas]

We need to separate the content of the layout (menu, footer). Part of the idea from #1738 can be used, but we need Jekyll to generate it instead of modifying the content after the page loads. This is necessary to achieve #1676.

[bjohansebas]

https://github.com/untra/polyglot I have found this plugin, it is probably a good option

edit: we don't need that plugin, I already found an elegant way to do it

[bjohansebas]

The only thing left is to move the i18n-alert (https://github.com/expressjs/expressjs.com/tree/gh-pages/_includes/notice)

Example: #1784