DOCSP-35807: Add new Flutter getById() method to Connect to App page

[cbullinger]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-35807

• Connect to App Services - Flutter SDK: Add new "Get App by ID" section with a tested, Bluehawked code example to document new getById() method, and update existing section with new behavior when opening an App on a background isolate.

Reminder Checklist

Before merging your PR, make sure to check a few things.

• Did you tag pages appropriately?
  • genre
  • meta.keywords
  • meta.description
• Describe your PR's changes in the Release Notes section
• Create a Jira ticket for related docs-app-services work, if any

Release Notes

• Flutter SDK
  • Atlas App Services/Connect to App Services: Add new "Get App by ID" section with tested, Bluehawked code example that documents how to access an App instance on a background isolate.

Review Guidelines

REVIEWING.md

[github-actions]

Readability for Commit Hash: 821291a

You can see any previous Readability scores (if they exist) by looking
at the comment's history.

Readability scores for changed documents:

• source/sdk/flutter/app-services/connect-to-app: Grade Level: 6.8, Reading Ease: 67.76

For Grade Level, aim for 8 or below.

For Reading Ease scores, aim for 60 or above:

Score	Difficulty
90-100	Very Easy
80-89	Easy
70-79	Fairly Easy
60-69	Medium
50-59	Fairly Hard
30-49	Hard
0-29	Very Hard

For help improving readability, try Hemingway App.

[dacharyc]

Overall, LGTM. Just a handful of Qs and a couple of nits you can feel free to disregard.

[docs-builder-bot]

✨ Staging URL: https://preview-mongodbmongodb.gatsbyjs.io/realm/master/

🪵 Logs

• job log: f6d4ce46dccc632b5ec1f74db72a987d02a80589