docs/install: Doc direct bootc raw and systemd creds

First, we're still in a situation where bib doesn't work with c9s.
Let's link to the existing docs for using `bootc install to-disk`
which *does* work (and fixes multiple other bugs at the same time too).

Second: Let's document using systemd credentials to inject a root
SSH key, because this works across every image we ship where
one can inject SMBIOS bits.  (But notably this doesn't work in
most production IaaS virt systems like KubeVirt, AWS etc. which
gets into cloud agents).

docs/install.md

@@ -26,6 +26,27 @@ supports generating disk images, including injecting user accounts.
 
 NOTE: this tool [does not yet work with centos stream 9](https://github.com/osbuild/bootc-image-builder/issues/20).
 
+## Generating a raw disk image that can be launched via virt tooling
+
+The above bootc-image-builder tool can generate disk images; however, a key part
+of the idea of `bootc` is that operating system images that use it are their
+own self-sufficient "baseline" installer.  So you can use this example:
+
+<https://github.com/containers/bootc/blob/main/docs/install.md#using-bootc-install-to-disk---via-loopback>
+
+to generate a raw disk image from the default container base image, or your own
+without any external tooling.
+
+If you choose not to include SSH keys or other credentials directly in your image,
+a useful pattern can often be to use [systemd credentials](https://systemd.io/CREDENTIALS/)
+to inject a SSH key for root.  The above page has this example for qemu:
+
+```bash
+-smbios type=11,value=io.systemd.credential.binary:tmpfiles.extra=$(echo "f~ /root/.ssh/authorized_keys 600 root root - $(ssh-add -L | base64 -w 0)" | base64 -w 0)
+```
+
+Unlike current bootc-image-builder, this flow works with current CentOS Stream 9.
+
 ## Installation using Anaconda
 
 Tools like