docs: Add guide to convert integrations to Swift

[philprime]

• Adds the guide how to convert an integration entrypoint from Objective-C to Swift to work on Refactor integration entry points in Swift #6840
• I used this to draft up ref: convert SentryViewHierarchyIntegration to Swift #7157 and ref: convert AutoBreadcrumbTrackingIntegration to Swift #7158 before manually reviewing and resolving issues (before creating the PRs)
• Can be used in a prompt for an LLM coding agent like this:

I need you to use the guide in @develop-docs/CONVERTING_INTEGRATION_TO_SWIFT.md to convert the integration in @Sources/Sentry/SentryAutoBreadcrumbTrackingIntegration.m to Swift. Instead of editing the Xcode project by yourself, tell me which files to create or delete.

• We will remove this guide from the develop docs after Refactor integration entry points in Swift #6840 is done

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Semver Impact of This PR

🟢 Patch (bug fixes)

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[philipphofmann]

LGTM, maybe we should consider following the agent skills format, which works with most AI dev dools, but we can also keep it as it is.