Feat: Support LLMs for i18n Translation + remove CrowdIn ads

### Have you read the Contributing Guidelines on issues?

- [x] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#issues).

### Description

Add i18n auto-translation support using LLMs via a flexible API.

### Has this been requested on Canny?

_No response_

### Motivation

Modern LLMs are better technical translators than the average CrowdIn human.

It would make sense to fully automate the translation flow in CI, and possibly deprecate CrowdIn ~support~ mentions in the future.

### API design

`docusaurus --translation-source-lang=en --translation-target-langs=fr,de --translation-llm-endpoint=[openai-compatible router]`

### Have you tried building it?

This should be a core feature.

### Self-service

- [ ] I'd be willing to contribute this feature to Docusaurus myself.