DOCSP-34729: Update Manage Subscriptions page with Subscribe API for Flutter

[cbullinger]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-34729

• Manage Subscriptions - Flutter SDK: Update page with new documentation for new .subscribe() and .unsubscribe() methods, similar to the other SDKs.

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
  • Sync Device Data/Manage Subscriptions: Update page to document the new Subscribe API with tested, Bluehawked code examples.

Review Guidelines

REVIEWING.md

[github-actions]

Readability for Commit Hash: e01540b

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/sync/manage-sync-subscriptions: Grade Level: 9.2, Reading Ease: 50.84
• source/sdk/react-native/sync-data/flexible-sync: Grade Level: 8.0, Reading Ease: 59.09

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.

[krollins-mdb]

LGTM, with some minor suggestions. I know some of these comments are on things that aren't part of what you updated, but it can be hard to tell with large page changes.

[docs-builder-bot]

✨ Staging URL: https://docs-mongodborg-staging.corp.mongodb.com/realm/docsworker-xlarge/master/

🪵 Logs

• job log: 59ddcc36eed9a6e666480d778cd60e9a5c80b2ab