Add Windows environment variable instructions for INFLUXDB3_AUTH_TOKEN

[Copilot]

Windows users were getting confused by the documentation which only provided Unix-style commands for setting the INFLUXDB3_AUTH_TOKEN environment variable. This was causing issues for Windows users trying to follow the getting started guide.

Before:
The documentation only showed:

export INFLUXDB3_AUTH_TOKEN=YOUR_AUTH_TOKEN

After:
The documentation now provides platform-specific instructions using a tabbed interface:

• macOS/Linux: export INFLUXDB3_AUTH_TOKEN=YOUR_AUTH_TOKEN
• PowerShell: $env:INFLUXDB3_AUTH_TOKEN = "YOUR_AUTH_TOKEN"
• CMD: set INFLUXDB3_AUTH_TOKEN=YOUR_AUTH_TOKEN (with trailing space and explanatory note)

The implementation follows the existing pattern used in the cloud-dedicated documentation, using Hugo shortcodes ({{< tabs-wrapper >}}, {{% tabs %}}, and {{% tab-content %}}) to create an interactive tabbed interface.

Since this change is made to the shared content file (/content/shared/influxdb3-get-started/setup.md), it automatically applies to both InfluxDB 3 Core and Enterprise documentation.

Fixes #6290.

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[jstirnaman]

@copilot Use the new code-placeholders syntax, which appends attributes to the code block fence--for example:

```bash { placeholders="YOUR_AUTH_TOKEN|YOUR_DATABASE_NAME" }

[Copilot]

@copilot Use the new code-placeholders syntax, which appends attributes to the code block fence--for example:

...

Updated to use the new { placeholders="..." } syntax as requested. All code blocks now use the fence attribute approach instead of the deprecated shortcode wrapper. The interactive placeholders are working correctly as shown in the updated screenshot.

Commit: 0ea9ca8

[jstirnaman]

👍