package/docker-containerd: rename package to containerd

package/docker-containerd: rename package to containerd

containerd is now an independent project from Docker.

This commit renames the Buildroot package from docker-containerd to containerd,
adding a entry in Config.in.legacy accordingly.

containerd is an industry-standard container runtime with an emphasis on
simplicity, robustness and portability. It is available as a daemon for Linux
and Windows, which can manage the complete container lifecycle of its host
system: image transfer and storage, container execution and supervision,
low-level storage and network attachments, etc.

https://containerd.io

Signed-off-by: Christian Stewart <christian@paral.in>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout:
 - fix alphabetical ordering in package/Config.in
 - also do rename in DEVELOPERS
 - squash in second patch
]