content: rewrite slices section

This rewrite introduces slices in terms of arrays,
before introducing make. I think it flows more nicely.

Fixes golang/go#13883
Fixes golang/go#14893

Change-Id: Idd0c6de29c0207d755cac673e580efee6c34a7b2
Reviewed-on: https://go-review.googlesource.com/21358
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
8 files changed