Add Mistral AI documentation.

[tzolov]

After the merge of #369 add

• Mistral AI Chat client docs under: https://github.com/spring-projects/spring-ai/tree/main/spring-ai-docs/src/main/antora/modules/ROOT/pages/api/clients
• Mistral Ai Embedding docs under: https://github.com/spring-projects/spring-ai/tree/main/spring-ai-docs/src/main/antora/modules/ROOT/pages/api/embeddings
• Add a mistral chat and embedding items in the catalog: https://github.com/spring-projects/spring-ai/blob/main/spring-ai-docs/src/main/antora/modules/ROOT/nav.adoc

[tzolov]

Resolved by 31d6194 and aa55a96