🩹[Patch]: Improve error handling and output error summary (#23)

## Description

This pull request makes a minor improvement to error handling in the
PowerShell documentation build script. Instead of writing errors as
errors, it now logs them as warnings with more context when markdown
help generation fails for a command.

* Improved error reporting by changing `Write-Error` to `Write-Warning`
with a descriptive message in `Build-PSModuleDocumentation.ps1`.

Author: MariusStorhaug

scripts/helpers/Build-PSModuleDocumentation.ps1

@@ -85,7 +85,7 @@
                     Error       = $_
                     ErrorString = $_.ToString()
                 })
-            Write-Error $_
+            Write-Warning "Failed to generate markdown help for $($command.Name): $_"
         }
     }
     Write-Host '::endgroup::'