Add the product hubs to the left nav after Elastic fundamentals

[florent-leborgne]

Summary

Adds the product hubs to the assembled left nav. Elastic Stack becomes a group right after Elastic fundamentals, with the Elasticsearch, Kibana, and Logstash hubs under it.

Follows the hub pages work in elastic/docs-content#7964 and the nav change in elastic/docs-content#8870. Relates to docs-content-internal#1561.

• config/navigation.yml: adds - toc: products after get-started in the narrative section.
• config/navigation.yml: moves deploy-manage up to follow products, so the order is Elastic fundamentals, Elastic Stack, Deploy and manage, then Solutions and use cases.
• config/navigation.yml: removes the docs-content://products phantom, the entry that declared the toc without showing it in the nav. The toc is now listed in the nav.
• config/navigation_preview.yml is unchanged. It keeps its own phantom and the static Products dropdown.

Merge order

Merge this PR last. The assembler build check fails until the docs-content PRs are merged. That failure is expected, so keep this PR as a draft until then.

1. elastic/docs-content#7964 adds the hub pages.
2. elastic/docs-content#8870 loads toc: products in the docs-content docset.
3. This PR adds the group to the assembled left nav.

The assembler builds the nav against the default branch of docs-content. Until step 2 lands, it can't find docs-content://products/, and the check reports "Could not find navigation node for identifier".

Test plan

• docs-builder assembler navigation validate passes with - toc: products
• In an assembler build, Elastic Stack appears right after Elastic fundamentals
• Elastic Stack expands to Elasticsearch, Kibana, and Logstash
• Deploy and manage follows Elastic Stack
• Hub pages keep a left sidebar in the assembled site

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

Tool(s) and model(s) used: Cursor, Claude Sonnet 5.5 (nav change and PR description)

[github-actions]

Docs preview (local build)

Handbook preview: https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/4477/