Improve Elasticsearch serverless APIs docs landing page

[leemthompo]

Attempts to:

• Add clear section headings
• Group related content logically
• Add descriptive summaries for each API set
• Improve navigation with direct links

ℹ️ I didn't touch this page but I think it probably needs updating too, should title be "{serverless-full} API conventions' rather than "Management API conventions"?

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[lcawl]

LGTM!