(maint) Fix Extension guide

[steviecoaster]

• Missing directive in nuspec
• Missing Export-ModuleMember in .psm1

Description Of Changes

• Missing <files> directive in nuspec
• Missing Export-ModuleMember in .psm1

Motivation and Context

The resultant package from the guide needs to work

Testing

• I have previewed these changes using the Docker Container or another method before submitting this pull request.

Change Types Made

• Minor documentation fix (typos etc.).
• Major documentation change (refactoring, reformatting or adding documentation to existing page).
• New documentation page added.
• The change I have made should have a video added, and I have raised an issue for this.
  • Issue #

Change Checklist

• Requires a change to menu structure (top or left-hand side)/
• Menu structure has been updated

Related Issue