DOCSP-36259: Add user.changes documentation for Flutter

[cbullinger]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-36259

• React to Changes - Flutter SDK: Add new Register a User Instance Change Listener section
• User Management - Flutter SDK: Add new Listen for User Changes section that links to the React to Changes page's new section

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
  • Realm Database/React to Changes: Add new Register a User Instance Change Listener section documenting how to listen for user changes with a tested, Bluehawked code example.
  • Manage Users: Add Listen for User Changes section with link to React to Changes page.

Review Guidelines

REVIEWING.md

[github-actions]

Readability for Commit Hash: b2dafa2

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/users: Grade Level: 11.0, Reading Ease: 40.65
• source/sdk/flutter/realm-database/react-to-changes: Grade Level: 7.9, Reading Ease: 59.5

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]

LGTM!

[docs-builder-bot]

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

🪵 Logs

• job log: a71edef2def18b2acaf0ea9bd7a901c6d102a293