[Serverless] Set observability onboarding as default page

[yngrdyn]

This PR set observabilityOnboarding as default page for serverless observability

Screen.Recording.2023-05-25.at.11.15.00.mov

[apmmachine]

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Just comment with:

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

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 6fbb4be

Metrics [docs]

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
securitySolution	400	404	+4
total			+6

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
securitySolution	480	484	+4
total			+6

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@elasticmachine merge upstream