AI Gateway [x.x]: Add documentation for Weaviate support

## Description

Document Weaviate as an additional embedding storage option alongside Postgres and Redis. Include basic setup, config example, and a short note on when to use it.

Reference: [https://weaviate.io/](https://weaviate.io/)

## DoD

* Weaviate added to [AI semantic Cache](https://developer.konghq.com/plugins/ai-semantic-cache/#vector-databases) plugin and [AI semantic similarity](https://developer.konghq.com/ai-gateway/semantic-similarity/) reference documentation
* Basic config example included to AI Semantic Cache plugin

## Information

https://konghq.atlassian.net/browse/AG-308
Person of contact: Wenchao Xiang

### Size

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