Add C# example for the AudioStreamGenerator code snippet

[NiftyHat]

Added a C# example for the AudioStreamGenerator code snippet.

[akien-mga]

Thanks for the contribution!

I pointed out some code style issues in the example which should be solved. The best way to do it is by amending the commit so that it stays as a single commit in this PR, which is the way we'd want to merge such a documentation improvement.
See PR workflow for instructions.

While at it, please also update the commit message to be more explicit (e.g. reusing what I put as PR title). "Update file" doesn't say what the update does so it's not a good informative commit message.

[NiftyHat]

I've made the changes but I think something went wrong the the amendment and now there are two commits instead of one. I did try to squash them but git wouldn't let me due to the commits having multiple parents :(

[akien-mga]

I squashed the commits for you, it wasn't trivial indeed after the latest changes, not sure how the Git history ended up exactly.

I also fixed a few formatting issues (trailing whitespace, [codeblocks] should be plural when using the language-specific subblocks, and < needs to be escaped as < in XML).

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉