Adds Stride3D, the open-source c# game engine

[tebjan]

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)

Stride is one of the largest non-profit open-source game engines, and I was surprised it was missing from this collection.

Adds the repo stride3d/stride to the list.

[Copilot]

Pull Request Overview

This PR adds the Stride3D engine to the existing list of open-source game engines.

• Inserts stride3d/stride into the items array in index.md.

Comments suppressed due to low confidence (1)

collections/game-engines/index.md:4

• Ensure the indentation of this list item matches the existing entries (e.g., two spaces before the dash) so the YAML-like list renders correctly.

 - stride3d/stride