Docs: add Claudexia as OpenAI-compatible LLM provider

[SeMax1337]

This PR adds documentation for Claudexia, a pay-per-token gateway that exposes Anthropic Claude and OpenAI GPT models through an OpenAI-compatible API. Aider already works with it out of the box via OPENAI_API_BASE + --model openai/...; this just documents how.

Files added:

• aider/website/docs/llms/claudexia.md — provider page following the same template as deepseek.md / openrouter.md (frontmatter, env-var bash blocks, model examples).

No code or model-list changes — pure docs.

Link: https://claudexia.tech

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