Fix `Clean` task in template

Description

Adds sane defaults for the output paths for PDF and static site to the template psake file.

Related Issue

• Resolves #13 (closed)

Motivation and Context

The Clean task is a prereq for most other tasks to function. If it fails, they never run.

How Has This Been Tested?

Locally, Manually.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.