Improve installation instructions

[svencan]

This adds the link to the releases in the installation sections, as people who skip the "run the demo" part tend to download the main branch instead of a release.

[TokisanGames]

Great, thanks. Please add this to the first line , or [build the plugin from source](https://github.com/TokisanGames/Terrain3D/wiki/Building-From-Source).

Generally I'm not so concerned about squashing, but if you were submitting to Godot they are religious about it. It will be good practice to learn and for fixing a change like this. Make the above change and commit. Then look at the PR workflow document to do an interactive rebase git rebase -i HEAD~3, change the first word of the second and third commits to fixup so it merges them all into one, then do a force push to overwrite the server branch.

[svencan]

Changed as requested and squashed the commits. Thanks for the hint!

[TokisanGames]

Great, thanks!