docs(kits): document differences from the legacy extensions

[IzaakGough]

Every kit's changelog said only "Initial release", and nothing told a user moving off an installed extension instance what would behave differently.

Each kit README gains a ## Differences from the <extension> extension section, and the changelog entry points at it. Twelve kits are covered. firestore-incremental-capture is untouched: this kit is still in the process of being implemented and tested.

Each section covers only what a migrating user acts on: settings renamed or newly defaulted, install-time validation that now fails at runtime, secrets that must exist by name, region and generation changes, data shape changes, and tooling that did not come across. Shared template differences (extension.yaml to npm package, params, HandlerContext, gen1 to gen2 boilerplate) are deliberately left out.

Verified by reading the extension and kit source for each pair, plus the extension.yaml params. Nothing was exercised against a live deploy, and each section says so where a claim depends on one.

Two decisions for a reviewer: whether the section belongs before ## API surface, where it is now, or higher up in each README; and whether to leave the firestore-incremental-capture changelog as it is.

[gemini-code-assist]

Code Review

This pull request updates the CHANGELOG.md and README.md files across multiple Firebase kits, including bigquery-firestore-export, delete-user-data, firestore-bigquery-export, firestore-bundle-builder, firestore-counter, firestore-genai-chatbot, firestore-send-email, firestore-translate-text, firestore-vector-search, rtdb-limit-child-nodes, speech-to-text, and storage-resize-images. The updates introduce a comprehensive 'Differences' section in each README to guide users migrating from the legacy Firebase Extensions to these repackaged npm packages, detailing changes in configuration, runtime behavior, and deployment regions. As there are no review comments provided, I have no feedback to offer.