docs: add FlagLint migration workflow

[Krishan27]

Summary

Adds FlagLint as an optional AST-based workflow in the existing LaunchDarkly-to-OpenFeature migration guide.

The existing manual IDE and text-search workflow remains unchanged. The new section shows how to:

• inventory supported LaunchDarkly call sites with scan
• summarize migration risk and readiness with audit
• preview safe OpenFeature call-site rewrites with migrate --dry-run

Scope

This change documents application source-code analysis and call-site migration only.

FlagLint does not migrate:

• flag definitions
• targeting rules
• environments
• backend state
• provider configuration

Verification

• Verified the documented commands against published flaglint@0.8.0
• Ran Vale on the modified MDX file
• Previewed the page locally with Mintlify
• Confirmed migrate --dry-run does not modify source files

I scoped this change to the existing v1 LaunchDarkly-to-OpenFeature migration guide. I'm happy to add equivalent guidance under v2 if the maintainers prefer a corresponding migration page there.

closes #408.

[Krishan27]

Thanks again, @markphelps I opened the focused documentation PR here: #407.

I kept the existing manual discovery workflow unchanged and added FlagLint as an optional AST-based workflow for scanning usages, assessing migration readiness, and previewing safe OpenFeature rewrites.

I scoped this change to the existing v1 LaunchDarkly-to-OpenFeature migration guide. I'm happy to add equivalent guidance under v2 if the maintainers prefer a corresponding migration page there.

[markphelps]

lgtm. thanks @Krishan27

[Krishan27]

Thanks @markphelps appreciate the approval. The Claude check looks unrelated to this docs change; Vale and ESLint both pass.