Add gdscript highlight in markdown code block

[at15]

Add grammar to highlight gd script in markdown.

The main use case I have is let claude/codex generate me a tutorial doc so that I can
follow it and write the scripts by myself instead of vibe everything.

[HolonProduction]

Just to make sure since you mentioned it in the description: Did you utilize AI to create this PR?

I don't think gd should inject GDScript highlighting, to keep things consistent with Github.

[at15]

Did you utilize AI to create this PR

Yeah, for writing the tmlanguage json. But I didn't find where/how to attribute that in CONTRIBUTING.md,

Changed to only allow gdscript didn't know github has highlight for gdscript, which is case insensitve. I only found gh's highlight for file, didn't find where they configure the fence tag though ...https://github.com/github-linguist/linguist/blob/1d7ac7ed569bd6edef5d0cfc73feea2573cb0e03/lib/linguist/languages.yml#L2411-L2418

extends Node

func _ready() -> void:
	print("Hello from gdscript")

extends Node

func _ready() -> void:
	print("Hello from gdscript")

extends Node

func _ready() -> void:
	print("Hello from gdscript")