xk6 topic: Updated links and description

[szkiba]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• This change is not self-promotion.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

Updated the xk6 topic link and description with up-to-date values.

[Copilot]

Pull Request Overview

This PR updates the xk6 topic metadata and description to point to the correct repository and documentation, and refreshes the description to highlight the xk6 tool.

• Updated github_url from grafana/k6 to grafana/xk6
• Updated url from k6.io/docs to the new Grafana docs path
• Revised the description to introduce xk6 and outline repository naming

Comments suppressed due to low confidence (2)

topics/xk6/index.md:3

• The display_name still references 'Grafana k6 extension'. Consider updating it to 'Grafana xk6 extension' for clarity and consistency with the topic.

display_name: Grafana k6 extension

topics/xk6/index.md:6

• [nitpick] The short_description could explicitly mention xk6 to better reflect its purpose, e.g., 'Command-line tool for building custom k6 extensions with xk6.'

short_description: Extensions for the Grafana k6.