Demo needs a warning for root usage and places binary in /usr/sbin

**How To Reproduce**

Visit https://docs.k0sproject.io/v0.9.1/ and follow demo [gif](https://docs.k0sproject.io/v0.9.1/img/k0s_demo.gif)

**Expected behavior**

The demo runs a command which downloads and runs arbitrary code as root from the internet. This is a model that should be marked as non-production use only explicitly in the docs. This is never something someone should actually do on a machine they care about. The demo also installs the binary in /usr/bin which violates the normalized usage of the [Filesystem Hierarchy Standard](https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard).

Typically, /usr/local is expected to be used for non-distro binaries and packages. /usr/**bin** in particular is for non-essential binaries and k0s would most probably belong in /sbin.

The appropriate directory is either /opt/k0s/sbin/ or /usr/local/sbin.

```root@ip-172-31-12-11:~# which k0s```

> /usr/bin/k0s

Start with the v0.9.1 documentation page and its linked k0s demo GIF. Update the demo documentation to warn that downloading and running code as root is non-production guidance, and correct the binary location to the proposed appropriate directory. Done means both safety guidance and filesystem placement are explicit in the demo instructions.

Written by the indexing model from the issue text.