DOCSP-39164: Flutter: Change baseURL during runtime

[cbullinger]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-39164

• Connect to App Services - Flutter SDK: New "Connect to a Specific Server" section with baseURL info. New "Connect to a Different Server During Runtime" subsection with info about the experimental API.

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: New "Connect to a Specific Server" section with baseURL info. New "Connect to a Different Server During Runtime" subsection with info about the experimental API.

Review Guidelines

REVIEWING.md

[github-actions]

Readability for Commit Hash: 0509f70

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: 7.2, Reading Ease: 66.74

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, but if you take either of my suggestions from the Kotlin PR, we might apply the same things here.

[docs-builder-bot]

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

🪵 Logs

• job log: ee4b327fdd4c0e725ac7151c1d5a65a3c08c092f