Add Linux, Windows, DEB, and RPM standalone agent upgrade commands

[Copilot]

Summary

Fixes #[Fleet]: Standalone Agent Upgrade Commands not displayed for Non-macOS

The standalone agent upgrade documentation only showed macOS commands. Added tabbed interface with upgrade commands for all supported platforms:

• macOS: /Library/Elastic/Agent/
• Linux: /opt/Elastic/Agent/
• Windows: C:\Program Files\Elastic\Agent\ with PowerShell admin instructions
• DEB/RPM: Direct command (binary in PATH via systemd install)

Uses same tab-set syntax as install-standalone-elastic-agent.md and paths from installation-layout.md.

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: GitHub Copilot (Claude)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Fleet]: Standalone Agent Upgrade Commands not displayed for Non-macOS</issue_title>
<issue_description>Description:
Upgrade standalone agents section only displays macOS upgrade commands. Upgrade commands for other operating systems are missing.

Link:
https://www.elastic.co/docs/reference/fleet/upgrade-standalone

Screen-shot:

Whats working fine

• Installation commands are available for all operating systems.

Expected Result
Upgrade commands for standalone agents should be available for all supported operating systems/artifacts. </issue_description>

<agent_instructions>Here's the file: https://github.com/elastic/docs-content/edit/main/reference/fleet/upgrade-standalone.md

Add Linux, Windows, DEB, and RPM instructions. An example of the syntax is available here: https://www.elastic.co/docs/reference/fleet/install-standalone-elastic-agent (https://github.com/elastic/docs-content/edit/main/reference/fleet/install-standalone-elastic-agent.md).</agent_instructions>

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

• Fixes [Fleet]: Standalone Agent Upgrade Commands not displayed for Non-macOS ingest-docs#1876

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