docs(kmp): Flag CocoaPods deprecation and document SPM linking rules

[bealexaraujo]

DESCRIBE YOUR PR

The Kotlin Multiplatform docs still present CocoaPods as the default way to supply Sentry Cocoa, with no deprecation notice. The Apple docs already mark CocoaPods as no longer supported, but KMP users have no reason to visit them. The Gradle plugin page also doesn't say that the plugin skips SPM linking whenever the Kotlin CocoaPods plugin is applied. Users with that setup can follow the docs and end up with no working path.

This PR:

• Adds a shared includes/kmp-cocoapods-deprecation.mdx notice (v9.19.1 was the last CocoaPods release, and the trunk becomes read-only on Dec 2, 2026). It's imported on the KMP and Compose Multiplatform getting-started pages, Manual Setup, and the Gradle plugin page instead of being copied into each one.
• Lists SPM before CocoaPods in the Gradle plugin's feature lists and marks CocoaPods as deprecated.
• Replaces the Manual Setup SPM section, which said you must use CocoaPods and SPM together. It now points to the Gradle plugin + SPM path, links the Cocoa compatibility table, and adds a short "Migrating From CocoaPods" section.
• Adds a "How Auto-Install and Linking Interact" section to the Gradle plugin page with a table of supported setups.

Fixes: #19539

IS YOUR CHANGE URGENT?

• Urgent deadline (GA date, etc.): YYYY-MM-DD
• Other deadline: 2026-12-02
• No deadline: Not urgent, can wait up to 1 week+

PRE-MERGE CHECKLIST

• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

LEGAL BOILERPLATE

I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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