[NIM] Migration Guide page: Add mention of certificate file

[thomasbnt]

Hello, for the migration guide, miss certifications files.

For example:

- Step 5 – Copy the original `appsettings.json` and `appsettings-agent.json` files from `RuntimeOld` to the new `Runtime`.
+ Step 5 – Copy the original `appsettings.json` and `appsettings-agent.json` files, and your certificate file if changed from `RuntimeOld` to the new `Runtime`.