Migration Guide --> General Information --> Case Studies

[Migration Guide Project](https://github.com/dotnet/docs/projects/128)
New TOC: #23215
***Leverage existing documentation if it exists elsewhere.**

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Gather Case Studies from both 1st Parties and 3d Parties and link to them from here.

Some Case studies have already been made public and should be easy such as...
- https://devblogs.microsoft.com/dotnet/azure-active-directorys-gateway-service-is-on-net-core-3-1/
- https://devblogs.microsoft.com/dotnet/migration-of-bings-workflow-engine-to-net-5/

Others may need to be reviewed for public consumption...
- https://devdiv.visualstudio.com/FirstPartyEngagements/_wiki/wikis/FirstPartyEngagements.wiki/10957/POP3-Service-An-Exchange-service-performance-case-study