Update index.md in docs to import required things in code example

[insumanth]

Imported

1. datetime from datetime
2. PositiveInt and BaseModel from pydantic

Change Summary

Through it seems as this is not required as people will know it when reading docs, Pydantic docs provides option to run examples which will result in error instead of the required behavior that the docs is trying to convey.

Error without correct imports	Correctly conveying what docs is trying to show

Related issue number

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @sydney-runkle

[insumanth]

please review

[codspeed-hq]

CodSpeed Performance Report

Merging #10229 will not alter performance

_{Comparing insumanth:patch-1 (7b5602b) with main (0672da1)}

Summary

✅ 17 untouched benchmarks

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[Viicos]

Thanks for the contribution.