docs: Add example authentication configurations

[euanh]

Motivation

Public container registries often have more complex authentication schemes than privately-hosted registries. For instance, they might use authentication tokens instead of passwords, or authentication might be a separate service from the registry. Users will benefit from being able to refer to examples which show suitable .netrc entries, with information about how to generate tokens and avoid common problems.

Modifications

Add example configurations for a number of popular container registries.

Result

New users can refer to concrete examples of .netrc stanzas for popular container registries, along with outline instructions explaining how to generate the necessary authentication tokens.

Test Plan

Manually tested the instructions.
Automated tests continue to pass, no new documentation warnings.

[heckj]

great examples!!!

A few nits - basically introducing each list of instructions under the headers, and one minor rewrite to tighten and make the call to action more explicit