slices: add examples

slices: add examples

For Clone, Grow, Clip, Concat, Contains, Repeat.

Fixes #66435

Change-Id: Ife8f61427e9cd18b7106c100de8f82f9d7840c9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/573255
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>