docs: add Claudexia as OpenAI-compatible provider

[SeMax1337]

Summary

Adds a docs page for Claudexia as an OpenAI-compatible provider example. Claudexia is a pay-per-token Claude API gateway (also Anthropic-compatible) — useful for users who want to access Claude models via Continue's existing openai provider by setting apiBase.

What's added

• New page: docs/customize/model-providers/more/claudexia.mdx — config.yaml + config.json snippets showing how to wire Claudexia as an OpenAI-compatible provider, plus example model IDs and a multi-model role example.
• docs/customize/model-providers/overview.mdx — Claudexia listed under Hosted Services (alphabetical).
• docs/docs.json — sidebar entry under "More Providers" (alphabetical).

Pure docs change — no source, schema, or registry edits. Formatting mirrors existing sibling provider docs.

More info: https://claudexia.tech

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Summary by cubic

Add docs for configuring Claudexia as an OpenAI-compatible provider, with YAML/JSON examples using the openai provider and apiBase set to https://api.claudexia.tech/v1. Also adds Claudexia to the providers overview and sidebar, with common model IDs and a multi-model roles example.

^{Written for commit 8462956. Summary will update on new commits.}

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[cubic-dev-ai]

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