[ECS] [GPU]: Documentation to enable GPU support for ECS Agent

### Community Note

ECS Agent currently supports GPU resources by configuring `ECS_ENABLE_GPU_SUPPORT` propoerty. However, ECS use guide is missing documentation to enable GPU support.

https://github.com/aws/amazon-ecs-init/blob/master/ecs-init/config/common.go#L86

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**Tell us about your request**
What do you want us to build?
- Documentation update on GPU support for ECS

**Which service(s) is this request for?**
- ECS
**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

**Are you currently working around this issue?**
How are you currently solving this problem?
- Enable GPU support by configuring `ECS_ENABLE_GPU_SUPPORT =true` in ecs.config file

**Additional context**
Anything else we should know?

**Attachments**
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Start with ecs-init/config/common.go at the referenced GPU-support setting and locate the ECS use guide. Document how to enable GPU support with ECS_ENABLE_GPU_SUPPORT in the ecs.config file, and verify the guide clearly covers this configuration.

Written by the indexing model from the issue text.