Add examples and package documentation to the configopaque package.

[mx-psi]

To make it easier for users to understand and use the configopaque package, we should

• Add a package-level comment explaining the rationale for using configopaque (see [proposal] Add string type alias for sensitive fields #5653 for details)
• Add a testable example with a configuration struct with fields using configopaque.String, []configopaque.String and/or map[string]configopaque.String and showcasing how it looks once marshaled.

[jackycsl]

Can you please assign me this issue?
Thanks.

[mx-psi]

@jackycsl Consider it assigned and feel free to work on it :) (can't assign to you on the Github UI because you are not part of the OTel Github org, but that's not a problem, you can work on this!)