Separating tasks from Secret concept

There are many contents in the (*very big*) Secret concept page which
are actually tasks. This PR proposes a separation of some contents
into separate tasks, so that we have a (hopefully) better organization
of the content and we make room for improvement to the concept itself.

Note that the creation of the `configmap-secret` directory (instead of
`secret`) is an intent to create a folder for both ConfigMap and Secret,
both of which are about configurations though there are quite some
differences between them.

Author: tengqm