[Bug] [Docs] Add docs for gz plugins

[hamishwillee]

Describe the bug

Add docs for #24441 - (gz: use server config file for loading world plugins) as per #24441 (comment) and https://github.com/PX4/PX4-user_guide/pull/3628/files#r1990392300

@dakejahl and @hamishwillee to talk

[hamishwillee]

@dakejahl Note also https://github.com/PX4/PX4-user_guide/issues/3641#issuecomment-2720541759

I was focusing on: https://docs.px4.io/main/en/sim_gazebo_gz/index.html
In particular in the section "Adding New Worlds and Models", subsection "Adding a World". There is a reference to a CMakeLists.txt file that should be modified to generate the targets. However, checking the git history I see that such list has been removed in commit 6dc39d9deb6f1de4c1bb97461193aba8ff679c59. Is it possible that the documentation is now outdated?

Is this something that can be fixed quickly or does it have to wait on this discussion?

[hamishwillee]

@dakejahl I'm off for two weeks. Are you at the point where this can be documented? If so, feel free to kick it off.

[dakejahl]

Yeah, just need to find the time to do it. I also want to take one more pass at my optical flow plugin because I realized I did something wrong and want to fix it first.

[hamishwillee]

I'm back. This is your nag!