[Logs onboarding] Add landing page and client-side routing

[ogupte]

Closes #154928.

Adds landing page and client-side routing. Also rearranges the logs_onboarding files into their own custom_logs dir.

[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: bf810ea

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
observabilityOnboarding	64	35	-29

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
observabilityOnboarding	64.2KB	28.8KB	-35.4KB

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
observabilityOnboarding	4.4KB	4.3KB	-59.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	17	19	+2
securitySolution	399	402	+3
total			+5

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	18	20	+2
securitySolution	479	482	+3
total			+5

History

• 💚 Build #124021 succeeded 866710e
• 💛 Build #123744 was flaky 01cbabc

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[yngrdyn]

Do we need this?

[yngrdyn]

Is the # needed?

[yngrdyn]

LGTM 🌟
just some minor comments