Document the new enum/plugins setup

[zotanmew]

I've been testing version 9.0.0-preview.7 on .NET 9.0 RC1 and I'm getting the following errors:

Failed executing DbCommand (10ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
      UPDATE jobs AS j
      SET status = 0
      WHERE j.status = 2

As this exception does not happen on .NET 8.0, with efcore.pg version 8.0.4, I assume that the enum mapping is not working correctly on this version.

I'm mapping the enums using NpgsqlDataSourceBuilder.MapEnum<T>().

[roji]

Yeah, there have been some changes in the area that impact the way enums need to be configured. I'll document this soon.