Add ernie 4.5 21b config

[Tobivi]

Hi GaiaNet Team,

This PR adds a new node configuration for the baidu/ERNIE-4.5-21B-A3B-Thinking model, using a Q2_K GGUF quantization.

This contribution includes:

• The config.json file pointing to the model on the Hugging Face Hub.
• A README.md with setup instructions.

This configuration has been tested and runs successfully on my local GaiaNet node as shown in the startup logs.

Thank you!