docs: Update HA documentation

[tico24]

Modifications

• Conform the page to the Kubernetes documentation standard (as outlined in our docs contributing guide).
• Drop references to unsupported argo versions.
• Highlight that in most cases, a single replica controller remains the best solution to HA workflows.
• Introduce the concept of leader elections to the user and the pros and cons therein. Especially at scale

Verification

make docs