[docs] add Mistral AI provider documentation(Do not merge)

[siddhantwaghjale]

Description

Type of Change

• New content addition

Motivation and Context

Add documentation support for Mistral AI Provider

Areas Affected

The model provider page

Screenshots

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using mkdocs serve
• Links in the documentation are valid and working
• Images/diagrams are properly sized and formatted
• All new and existing tests pass

Additional Notes

This PR is related to: strands-agents/sdk-python#284
The test to check documentation for this doc are failing as the Mistral model in strands-sdk is not merged yet
Getting this error:

ERROR   -  Error reading page 'api-reference/models.md':
ERROR   -  Could not collect 'strands.models.mistral'

Aborted with a BuildError!```

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[mehtarac]

LGTM

[mehtarac]

Can we add this model provider to the https://github.com/strands-agents/docs/blob/main/mkdocs.yml#L84 along with the other model providers?