docs : added migration guide from @solana/kit to gill

[NVN404]

Problem

add guide page that demonstrates the simple process of migrating from @solana/kit to gill

we already have a gill vs @solana/kit comparison guide, but its not the same as a migration guide

Summary of Changes

• Added a new Migration Guide page for @solana/kit → gill.
• Documented equivalent flows with examples
• Highlighted key improvements and simplified steps with gill.

took reference from -
https://www.solanakit.com/docs/upgrade-guide

Fixes #200

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Latest commit: 6e2174d

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[nickfrosty]

This is basically the same thing as the existing comparison doc, not an actual migration guide

[NVN404]

I have updated the migration guide , can you please review it