docs(agents): add MistralChatClient to supported LLMs

[Rishabh-git10]

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Description

Adds MistralChatClient to the list of supported LLMs in the Dapr Agents Core Concepts page. This documents the new native Mistral SDK integration added to the dapr-agents Python SDK.

Issue reference

Accompanies implementation issue: dapr/dapr-agents#230

[sicoyle]

amazing - thank you so much for your docs contribution on the feature you enabled! 🚀

[Rishabh-git10]

@sicoyle I've added a commit to document the .prompty configuration support for the Mistral client. This accompanies the implementation just opened in dapr/dapr-agents#593.
Ready for another look when you have a moment!

[nelson-parente]

Pre-review for @marcduiker handoff: LGTM with nits.

• Adds MistralChatClient to the supported LLMs list and includes a Prompty template example for Mistral configuration.
• The .prompty file example is well-formatted and consistent with the existing docs style.
• Nit: the new #### Using Prompty Templates subsection is placed inside the LLM section but reads as a general feature — consider whether this should be its own top-level section or cross-reference an existing Prompty page if one exists.
• Nit: mistral-large-latest as the default model name may go stale as Mistral updates their naming. A note like 'check Mistral docs for current model names' would be helpful.
  Nothing blocking — Marc, your call.

[Rishabh-git10]

Thanks for the review @nelson-parente. I've addressed the nits.