doc: Use external-unstable again in getting started guide #1800

Job	Run time
Check if committed Helm charts are up to date	44s
Check if committed README is the one we would render from the available parts	23s
Run Cargo Deny (advisories)	39s
Run Clippy	31s
Run Cargo Deny (bans licenses sources)	37s
Run RustDoc	1m 1s
Run Rustfmt	11s
Run Cargo Tests	34s
Run Cargo Udeps	1m 24s
Select target helm repository based on action trigger	1s
All tests passed	1s
Package Charts, Build Docker Image and publish them - ubuntu-latest	10m 25s
Package Charts, Build Docker Image and publish them - ubicloud-standard-8-arm	10m 0s
Build and publish manifest list	39s
Run the OpenShift Preflight check on the published images	13s
	27m 23s

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