docs/getting-started.md: massive update

The previous documentation was too much forcusing on the Go API and not useful
for users who are not interested in implementing their own containerd client.
It was also recommending the deprecated way (cri-containerd-*.tar.gz) to install
containerd and its dependencies.

The new documentation recommends the current official way to install containerd,
and provides several links for end users.

This will replace the content of https://containerd.io/docs/getting-started/
after merging the containerd/containerd.io PR 120.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

docs/cri/installation.md

@@ -1,3 +1,14 @@
+This documentation was merged into [`../getting-started.md`](../getting-started.md).
+Please update your bookmark.
+
+
+- - -
+<!-- TODO: remove in containerd 2.0 -->
+<details>
+<summary>Show the original content (<strong>DEPRECATED</strong>)</summary>
+
+<p>
+
 # Install Containerd with Release Tarball
 This document provides the steps to install `containerd` and its dependencies with the release tarball, and bring up a Kubernetes cluster using kubeadm.
 
@@ -130,3 +141,6 @@ systemctl daemon-reload
 Now you should have properly installed all required binaries and dependencies on each of your node.
 
 The next step is to use kubeadm to bring up the Kubernetes cluster. It is the same with [the ansible installer](../../contrib/ansible). Please follow the steps 2-4 [here](../../contrib/ansible/README.md#step-2).
+
+</p>
+</details>