flutter: improve app start and frames tracking docs

[buenaflor]

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PRE-MERGE CHECKLIST

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• [ ] 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

Look, 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.

EXTRA RESOURCES

• Sentry Docs contributor guide

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 20, 2024 7:34pm

2 Skipped Deployments

| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **changelog** | ⬜️ Ignored ([Inspect](https://vercel.com/sentry/changelog/5yd4adBknso64LZZECs1zAU3Rgys)) | [Visit Preview](https://changelog-git-feat-flutter-app-start.sentry.dev) | | Nov 20, 2024 7:34pm | | **develop-docs** | ⬜️ Ignored ([Inspect](https://vercel.com/sentry/develop-docs/DZq6XNAo9WUpqte3Tj9FV7Xo7FQN)) | [Visit Preview](https://develop-docs-git-feat-flutter-app-start.sentry.dev) | | Nov 20, 2024 7:34pm |

[codecov[bot]]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) :arrow_down:. This is within the configured threshold :white_check_mark:

Detailed changes

| Bundle name | Size | Change | | ----------- | ---- | ------ | | sentry-docs-server-cjs | 10.28MB | 9 bytes (-0.0%) :arrow_down: | | sentry-docs-client-array-push | 9.29MB | 6 bytes (-0.0%) :arrow_down: |