Move egame dev example to gitlab-examples

What does this MR do?

As part of the linked issue, we can clean up some old examples by moving the docs into the source project. This was a blog post originally, but was merged into the main docs repo before it got published on the blog site, I believe. It has not been maintained since, so it would be better to keep this with the original project, which is a bit more "timeless" and has less maintenance burden.

Related issues

• Related to #249525 (comment 482229905)
• Related to https://gitlab.com/gitlab-examples/gitlab-game-demo

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• If you have Developer permissions or higher:
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• Update the permissions table.
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