docs: add llm-grok (#629)

Adds`llm-grok` xAI API (https://github.com/Hiepler/llm-grok) to the plugin directory.

!stable-docs

docs/plugins/directory.md

@@ -27,6 +27,7 @@ These plugins can be used to interact with remotely hosted models via their API:
 - **[llm-reka](https://github.com/simonw/llm-reka)** supports the [Reka](https://www.reka.ai/) family of models via their API.
 - **[llm-perplexity](https://github.com/hex/llm-perplexity)** by Alexandru Geana supports the [Perplexity Labs](https://docs.perplexity.ai/) API models, including `llama-3-sonar-large-32k-online` which can search for things online and `llama-3-70b-instruct`.
 - **[llm-groq](https://github.com/angerman/llm-groq)** by Moritz Angermann provides access to fast models hosted by [Groq](https://console.groq.com/docs/models).
+- **[llm-grok](https://github.com/Hiepler/llm-grok)** by Benedikt Hiepler providing access to Grok model using the xAI API [Grok](https://x.ai/api).
 - **[llm-anyscale-endpoints](https://github.com/simonw/llm-anyscale-endpoints)** supports models hosted on the [Anyscale Endpoints](https://app.endpoints.anyscale.com/) platform, including Llama 2 70B.
 - **[llm-replicate](https://github.com/simonw/llm-replicate)** adds support for remote models hosted on [Replicate](https://replicate.com/), including Llama 2 from Meta AI.
 - **[llm-fireworks](https://github.com/simonw/llm-fireworks)** supports models hosted by [Fireworks AI](https://fireworks.ai/).