Add description for o3de-extras backports

[jhanca-robotecai]

As discussed in the TSC meeting, we backported two Gems in o3de-extras repository (SimulationInterfaces and ROS2) in o3de-extras from 2510.0 to 2505.1 for our internal work. We can:

1. Keep it on our fork
2. Push it to the upstream repo (the community could find our work; we could use the upstream)
3. Make the official release of the Gems to make them available for the community via GUI (the best for the community).

We would like to go with option 3.

I decided to write an official document about it, to summarize the process and formalize it for the reuse. The only problem that needs to be solved is the documentation. In the future, we should also solve a problem with the versioning (we do not have it now, as 2510 was not released, hence we will bump the version number for the 2510 release).