SqlServerDsc: Automatically document public functions on build

[johlju]

The PR #1769 introduced public functions that are exported by the module. These public functions can be used directly from the command line, but the commands are also used by the DSC resources (currently resource SqlDatabasePermission).

BUt for users to use the public functions they should be documented as markdown files to the WikiSource folder so they can be published to the GitHub repository Wiki.

Suggest make a task that uses PlatyPS (there is already one that might be tweaked in Sampler), and there is script here that can be used as inspiration:
https://github.com/dsccommunity/ActiveDirectoryDsc/blob/main/source/Modules/ActiveDirectoryDsc.Common

The task should be in either Sampler or DscResource.DocGenerator - but can be be implemented here before it is moved if it is easier.