Use .example domain in samples

[wenz]

As part of DuendeSoftware/samples#209, we replaced "real" domains in the sample data (such as alice.com, bob.com, email.com) with alice.example.com, bob.example.com, and example.com.
After reading RFC 2606, I believe that the .example domain is an even better choice, so I've created a PR that changes that: what originally was email.com will become email.example, and alice/bob.com will turn into alice/bob.example.
This PR also changes "docs.example.come" (!) into "docs.example" in the MCP sample.

[wenz]

PR: DuendeSoftware/samples#277