docs: add FAQ item for existing users

[justlevine]

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• I have read the Contribution Guidelines.
• I have read the Development Guidelines.
• My code is tested to the best of my abilities.
• My code passes all lints (ESLint etc.).
• My code has detailed inline documentation.
• I have updated the project documentation as needed.

[Copilot]

Pull request overview

This PR adds a new FAQ entry addressing what happens to existing users when the OneAccess plugin is configured, and removes trailing whitespace throughout the README.md file for cleaner Markdown formatting.

• Adds a new FAQ item titled "What happens to my existing users?" explaining that pre-existing users continue to exist and are managed centrally once configured
• Removes trailing double spaces from the end of list items and sentences throughout the document (Markdown formatting cleanup)

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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[justlevine]

@up1512001 PTAL at ### What happens to my existing users? and lmk if it's accurate or if there are changes required.

(The rest of the diff is just removing trailing whitespaces)