Add: Announcement post for 15.0-RC3

[2TallTyler]

Long social message:

Third try's the charm? We hope so; here's our third release candidate for OpenTTD 15.

Most notably, the game no longer forgets your script configuration, and we've also fixed some rare crashes.

Help us test our work!
Full news: https://www.openttd.org/news/2025/12/20/openttd-15-0-rc3

Short social message:

OpenTTD 15.0-RC3 now available on Steam / our website.
Third try's the charm?

https://www.openttd.org/news/2025/12/20/openttd-15-0-rc3

Image:

News-15.0-rc3.zip