[Observability Onboarding] Link all “Add data” buttons in observability to onboarding landing page

[thomheymann]

Resolves #178944

📓 Summary

All "Add data" links within Observability link to /app/observabilityOnboarding when on serverless observability.

✔️ Acceptance criteria

All "Add data" links within Observability link to /app/observabilityOnboarding when on serverless observability:

• Infrastructure > Inventory
• Infrastructure > Hosts

[ghost]

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• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• /oblt-deploy-serverless : Deploy a serverless Kibana instance using the Observability test environments.
• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

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[cauemarcondes]

LGTM

[flash1293]

LGTM, the question is still whether these should link to the general guide as well:

Definitely out of scope for this issue though - let's capture that separately.

[flash1293]

@thomheymann I think this can target main directly as it makes sense even without the new onboarding page