Move each section in the menu bar to have individual pages

[sshkhr]

Current setup for all the site/_pages i.e. site/_pages/summerschool.html, site/_pages/archivesummerschool.html, site/_pages/recurringschool.html generates one big page with query filters to display individual schools.

This is not great for several reasons e.g. loading time, generating school based meta tags for social media.

We can use jekyll-datapage_gen plugin and liquid templating to generated individual pages for each school, since the list is fixed at build time.