Leverage Update-Markdownhelp when generating docs

[Dan Tsekhanskiy]

Description

This is more of a nit FR, but it would be performant to leverage the platyPS Update-MarkdownHelp function over the New-MarkdownHelp function when generating documentation.

Update-MarkdownHelp doc: https://github.com/PowerShell/platyPS/blob/master/docs/Update-MarkdownHelp.md
New-MarkdownHelp doc: https://github.com/PowerShell/platyPS/blob/master/docs/New-MarkdownHelp.md

Describe the solution you'd like

If the doc already exists for a given function, leverage Update-MarkdownHelp function over the New-MarkdownHelp function. This can be done with a PowerShell foreach loop and nested conditional.

Describe any alternatives you've considered

New-Markdownhelp technically works, but it seems unnecessary to generate entirely new documentation files every time this module is built, especially if the function(s) have not been updated.

Additional context

See platyPS module documentation here: https://github.com/PowerShell/platyPS