[zh] Add migration/_index.md

[my-git9]

zh: Add content/zh/docs/migration/_index.md

[svrnm]

This page will need to be updated to conform to opentelemetry.io/docs/contributing/localization/#translation-tips, in particular header IDs are missing.

@my-git9 please take a look

[svrnm]

@my-git9 would you be able to take a look?

[chalin]

Given that @open-telemetry/docs-zh-approvers have already approved, I'll go ahead and merge this. A followup PR can be submitted to add heading IDs if necessary. ✨