Fix copy elision

Use the new generator to create the index

Use a generator instead of relying on empty index templates to point at the layout file

While true, at first glance it appeared this was a necessary evil because I couldn't find a way to generate a set of pages from one template. As it turns out, Jekyll does support generators, so this patch isn't necessary.

Fix indentation

See D7130 and associated stack.

In D7065#170272, @Fabien wrote:

I don't see the value of this diff. IMO a shorter URL (3 chars !) is not a strong case and this:

• Makes the url inconsistent with the file system scheme,
• Will make it more complicated to add translations, if someone is willing to do so.

Rebase (doesn't really depend on D7065)