Improve documentation for Tailscale + K3s with an example ACL

[clouedoc]

Is your feature request related to a problem? Please describe.

I was stuck debugging a connectivity issue on my newly-created cluster for multiple days.
It turns out the Tailscale ACL I written was insufficient.
It looked like #8372 for a long while.

Describe the solution you'd like

An example Tailscale ACL in the k3s documentation.

Describe alternatives you've considered

• Taking a networking course
• Banging my head on the table (didn't help)
• Asking my friends for moral support

Example ACL

I attached an excerpt from my ACL which might help my fellow Tailscale+k3s enthusiasts.

Some information to know before reading:

• My cluster's nodes get auto-assigned the "skynet" tag.
• PodCIDR is 10.142.0.0/16 and ServiceCIDR is 10.143.0.0/16
• What was missing for me was to approve connections from the above subnets. I think I could skip the ServiceCIDR but I don't want to run into this issue again!
• "grafana:80" is just a service running on another machine connected to the network. I don't want skynet to escape...

{
  acls: [
    {
      action: 'accept',
      src: ['tag:skynet', '10.142.0.0/16', '10.143.0.0/16'],
      dst: ['10.142.0.0/16:*', '10.143.0.0/16:*', 'tag:skynet:*'],
    },
  ],
  autoApprovers: {
    routes: {
      '10.142.0.0/16': ['tag:skynet'],
      '10.143.0.0/16': ['tag:skynet'],
    },
  },
  tagOwners: {
    'tag:skynet': ['autogroup:admin'],
  },
  tests: [
    {
      src: 'tag:skynet',
      accept: ['10.142.0.1:6443', '10.143.0.1:6443', 'tag:skynet:6443'],
      deny: ['grafana:80'],
    },
    {
      src: '10.142.0.1',
      accept: ['10.142.0.1:6443', '10.143.0.1:6443', 'tag:skynet:6443'],
      deny: ['grafana:80'],
    },
    {
      src: '10.143.0.1',
      accept: ['10.142.0.1:6443', '10.143.0.1:6443', 'tag:skynet:6443'],
      deny: ['grafana:80'],
    },
  ],
}

[brandond]

cc @manuelbuil - I know we'd intentionally avoided getting too fancy with the example tailscale config in the docs. Not sure if this is something we want to entertain or not.