docs: add how-to for cert management

[steverfrancis]

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[nberlee]

Default kubeconfig is 1 year, but the doc is incorrect about talosctl config new which generates 10 years by default:

$ talosctl config new --help
  talosctl config new [<path>] [flags]

Flags:
      --crt-ttl duration   certificate TTL (default 87600h0m0s)

[smira]

Default kubeconfig is 1 year, but the doc is incorrect about talosctl config new which generates 10 years by default:
Flags:
--crt-ttl duration certificate TTL (default 87600h0m0s)

This feels like a bug actually, I will take a look

[smira]

/m