DnsServerDsc: The base classes need to be documented

[johlju]

We need to document when and how the base classes should be used. We need to document the patterns used.

[johlju]

This should be documented in the WIki by adding markdown files here: https://github.com/dsccommunity/xDnsServer/tree/main/source/WikiSource

They should also be referenced from the CONTRIBUTING.md.