Add mutation observer example to Azure AD B2C Javascript Docs

[jamescrowley]

The documentation mentions mutation observers in passing but doesn't explain why or give an example of how to use them.

We probably need confirmation from the team that what I'm proposing here is considered 'supported' but this is the only way I have found to ensure we can access DOM elements that Azure AD B2C has added to the document.

[prmerger-automator]

@jamescrowley : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

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articles/active-directory-b2c/javascript-and-page-layout.md	✅Succeeded

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[ttorble]

@garrodonnell

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
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