Added DispatcherQueueExtensions docs, deprecated DispatcherHelper

[Sergio0694]

Fixes #384 and #318

What changes to the docs does this PR provide?

• Added docs for DispatcherQueueExtensions (including migration from DispatcherHelper)
• Deprecated docs for DispatcherHelper

PR Checklist

Please check if your PR fulfills the following requirements:

• Correctly picked the right branch to base the change off (master for new features, live for typos/improvements)
• For new pages, used the provided template
• For new features, added an entry in the Table of Contents
• Ran against a spell and grammar checker
• Contains NO breaking changes

[opbld32]

Docs Build status updates of commit 9298459:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/extensions/DispatcherQueueExtensions.md	✅Succeeded	View
docs/helpers/DispatcherHelper.md	✅Succeeded	View
docs/toc.md	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

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[opbld31]

Docs Build status updates of commit 7618c40:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/extensions/DispatcherQueueExtensions.md	✅Succeeded	View
docs/helpers/DispatcherHelper.md	✅Succeeded	View
docs/toc.md	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[michael-hawker]

Couple of suggestions

[opbld32]

Docs Build status updates of commit ff634b4:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/extensions/DispatcherQueueExtensions.md	✅Succeeded	View
docs/helpers/DispatcherHelper.md	✅Succeeded	View
docs/toc.md	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[opbld31]

Docs Build status updates of commit 7578feb:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/extensions/DispatcherQueueExtensions.md	✅Succeeded	View
docs/helpers/DispatcherHelper.md	✅Succeeded	View
docs/toc.md	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[michael-hawker]

Thanks @Sergio0694!