Adjust the book with version 0.8.0

[thekuwayama]

Hi! This PR updates the book.

• adjust links that point into the no longer existent "generic" directory
• quinn::ClientConfigBuilder, ServerConfigBuilder and EndpointBuilder have already been removed (#1213), so adjust
• quinn::Certificate have already been removed (#1214), so adjust

I referred to quinn/examples/insecure_connection.rs, quinn/examples/client.rs and quinn/examples/server.rs to adjust the book.

[djc]

This looks great, thanks! A couple of ideas riffing on the same theme:

• Should we fix the links to docs.rs to point to specific versions rather than latest?
• Would it be viable to use skeptic to compile-test these code samples?