Updates after the removal of Scene3D plugin

[j-rivero]

Fixes: https://github.com/gazebosim/garden-tutorial-party/issues/2015

Following https://github.com/gazebosim/gz-gui/issues/484#issuecomment-1242525747

Updates for https://gazebosim.org/docs/garden/sdf_worlds and the example SDF file:

• Use gz instead of ign a25b01bde0e69653fb40af67f2eec8d5494d2db2
• Update world_demo.sdf after the removal of Scene3D dd5db60ef8335639938de7e6ae190a8e6f7b0109
• Update instructions after the removal of Scene3D 3f2366b49b3a1b577aa23dd57dddc8f364e23667

[azeey]

@j-rivero would you be able to address the feedback?

[azeey]

@j-rivero mind taking a look?

[azeey]

We'll go ahead and close this since most of the proposed changes have already been implemented.