Revive AsPlainText storage conversion

[lipkau]

Attribution

This PR revives and completes the work originally contributed by @empty03 in #178.

The first commit in this PR, d59f2c4 (SwitchParameter -AsPlainText), is cherry-picked from @empty03's original contribution and keeps their original author metadata:

empty03 57891206+empty03@users.noreply.github.com

The follow-up commit only updates the implementation for the current codebase, fixes edge cases, and adds tests/docs so the original contribution can be merged safely.

Summary

• revives original PR ConvertTo-StorageFormat add switch parameter -AsPlainText #178 by @empty03 with the original contributor commit preserved
• adds ConvertTo-ConfluenceStorageFormat -AsPlainText for publishing literal text containing Confluence wiki markup characters
• adds unit coverage, integration-suite coverage, command docs, and changelog attribution

Validation

• Invoke-Pester -Path 'Tests/Functions/ConvertTo-StorageFormat.Tests.ps1'
• Invoke-Build -Task Lint
• Invoke-Build -Task Build, Test

Notes

• replaces stale PR ConvertTo-StorageFormat add switch parameter -AsPlainText #178 with a current branch based on master
• live Cloud/Data Center integration tracks should still validate the Confluence API behavior in CI/manual integration runs