Tests: Build-PSBuildMarkdown

[HeyItsGilbert]

Add Pester 6 integration tests for Build-PSBuildMarkdown.

Scenarios to cover:

• Markdown docs are generated under the expected DocsPath
• Requires platyPS
• Depends on shared fixture module (see Tests: Create shared fixture module #97)

Target Pester 6.0.0 with classic Should syntax — see notes on #17. Parent issue: #17