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

[tablackburn]

Done, in tests/Build-PSBuildHelp.tests.ps1. Closing late: this should have closed with #150 in
#173, and the map said it would — but that pull request's description said only
Closes #150. Closes #151., so the sub-issues were never picked up. My omission.

Every scenario this issue asked for is covered, against the shared fixture from #97:

Asked for	Covered by
Markdown generated under the expected DocsPath	creates the locale directory under the docs path, writes one markdown file per exported command
Depends on the shared fixture module	the whole context runs on PSBuildTestFixture via New-PSBuildDocsScenario

Plus coverage the issue did not anticipate, because the PlatyPS 1.x migration made it necessary:
does not document private functions, produces markdown carrying the 1.x schema marker, keeps the markdown directly under the locale directory (1.x nests output under <ModuleName> and the
function flattens it back), writes a module landing page named for the module, and replaces the landing page on rebuild rather than leaving it stale.

One deviation from the issue text worth noting: it says "Requires platyPS". That is now
Microsoft.PowerShell.PlatyPS 1.x, and it is no longer a required module — see #105 and the
migration guide.