chore(model): add athene-v2-chat to the gallery

[mudler]

Description

This pull request includes a significant update to the gallery/index.yaml file, adding a new model entry for Athene-V2-Chat. The most important changes are as follows:

Addition of new model entry:

• Added a new model entry named athene-v2-chat with detailed description and multiple URLs. This model is described as an open-weights LLM comparable to GPT-4o, excelling in chat, math, and coding, and trained through RLHF with Qwen-2.5-72B-Instruct as the base model.
• Included an override section specifying the model parameter as Athene-V2-Chat-Q4_K_M.gguf.
• Added file details for Athene-V2-Chat-Q4_K_M.gguf with its SHA256 checksum and URI.

Notes for Reviewers

Signed commits

• [ ] Yes, I signed my commits.

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