Add description metadata to first-level index pages

[Copilot]

Summary

This PR adds description metadata to the frontmatter of all 10 first-level index.md files in the repository to improve SEO and provide clear page summaries.

Changes

Added concise, informative descriptions to the following first-level index pages:

• cloud-account/index.md - Description about managing Elastic Cloud account settings and preferences
• deploy-manage/index.md - Description about deploying and managing Elastic environments
• explore-analyze/index.md - Description about exploring and analyzing data with Kibana
• extend/index.md - Description about extending the Elastic Platform (also added frontmatter structure)
• get-started/index.md - Description about Elastic fundamentals and getting started
• manage-data/index.md - Description about data ingestion and management in Elasticsearch
• reference/index.md - Description about Elastic Stack reference documentation
• release-notes/index.md - Description about release notes and changelogs (also added frontmatter structure)
• solutions/index.md - Description about Elastic solutions for search, observability, and security
• troubleshoot/index.md - Description about troubleshooting resources and support

Implementation Details

• All descriptions follow the YAML multi-line format consistent with the root index.md file
• Descriptions are concise (2-3 sentences) and accurately reflect the content of each page
• Two files (extend/index.md and release-notes/index.md) previously had no frontmatter and now have proper YAML frontmatter blocks
• All YAML syntax has been validated

Related Issue

Addresses the requirement outlined in elastic/docs-builder#1559

Example

Before:

---
applies_to:
  stack: ga
  serverless: ga
---

After:

---
applies_to:
  stack: ga
  serverless: ga
description: Learn how to ingest, store, and manage data in Elasticsearch. Understand
  indices, mappings, text analysis, data lifecycle management, and migration between
  clusters.
---

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add description metadata to first-level index pages</issue_title>
<issue_description>Most Markdown docs in this repository still lack a description: in the frontmatter. We should at least add a description for the first level pages, that is, the index.md pages inside each of the docs folders.

Related issue: elastic/docs-builder#1559</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #3394

---

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[github-actions]

🔍 Preview links for changed docs

• cloud-account/index.md
• deploy-manage/index.md
• explore-analyze/index.md
• extend/index.md
• get-started/index.md
• manage-data/index.md
• reference/index.md
• release-notes/index.md
• solutions/index.md
• troubleshoot/index.md

[florent-leborgne]

Simple progress! Nice!

[leemthompo]

LGTM

[yetanothertw]

LGTM, thank you