Spring-ai Mistral AI support documentations

Comment From: tzolov

Thanks @ricken07 !

Additional changes made: - fix a MistralAiApi (and OpenAiApi) issue just introduced by Mistral API - change the auto-config properties prefix from spring.ai.mistral. to spring.ai.mistralai. - minor doc references updates.

rebased, squashed and merged at 31d6194d9debb079ee3c3e30ee4d15abfa7c4db1