Add migration guide to v3.0

[twsouthwick]

No description provided.

[twsouthwick]

@AlfredHellstern can I get your eyes on this doc?

[learn-build-service-prod]

Learn Build status updates of commit ab8afe3:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/general/diagnosticids.md	✅Succeeded
docs/general/overview.md	✅Succeeded
docs/migration/migrate-v2-to-v3.md	✅Succeeded
docs/open-xml-sdk.md	✅Succeeded
docs/toc.yml	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit eef7dcc:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/general/diagnosticids.md	✅Succeeded
docs/general/overview.md	✅Succeeded
docs/migration/migrate-v2-to-v3.md	✅Succeeded
docs/open-xml-sdk.md	✅Succeeded
docs/toc.yml	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 0b1ba55:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/general/diagnosticids.md	✅Succeeded
docs/general/overview.md	✅Succeeded
docs/migration/migrate-v2-to-v3.md	✅Succeeded
docs/open-xml-sdk.md	✅Succeeded
docs/toc.yml	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 9b7ba89:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/general/diagnosticids.md	✅Succeeded
docs/general/overview.md	✅Succeeded
docs/migration/migrate-v2-to-v3.md	✅Succeeded
docs/open-xml-sdk.md	✅Succeeded
docs/toc.yml	✅Succeeded
docs/what-s-new-in-the-open-xml-sdk.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel