Update FakeItEasy docs for 8.0.0 (remove Provide / builder)

[tillig]

Summary

Updates the FakeItEasy integration documentation to match the 8.0.0 changes in Autofac.Extras.FakeItEasy, where the AutoFake.Provide<T>() methods and the builder constructor parameter were removed in favor of the configureAction constructor argument.

Changes

• Rewrote the "Configuring Specific Dependencies" section to use the configureAction constructor argument (mirroring the Moq integration docs) instead of the removed Provide() methods.
• Added a "Migrating from 7.0.0" section with before/after examples for instance registrations, implementation registrations, and the removed builder parameter.
• Fixed a pre-existing error: the "Options for Fakes" example referenced a constructor parameter named onFakeCreated, but the actual parameter is configureFake.

Related

Companion to the library PR: autofac/Autofac.Extras.FakeItEasy#30