Use native folder-derived Zensical navigation #189

Need

The documentation site still defines a large manual nav array in src/zensical.toml, while the intended shared default is Zensical's native navigation derived from folders and filenames. The current index-maintenance script, tests, and contributor guidance are coupled to that manual array.

Expected outcome

The documentation folder structure is the navigation source of truth. Index pages appear first and remaining pages sort alphabetically without maintaining a parallel navigation list.

Acceptance criteria

• Remove the explicit nav array from src/zensical.toml.
• Preserve the current documentation hierarchy through folders and index.md files.
• Update .github/scripts/Update-DocumentationIndex.ps1 so it no longer requires or derives order from configured navigation.
• Update the corresponding Pester tests and fixtures.
• Update CONTRIBUTING.md and src/docs/Ways-of-Working/Workflow-Stages/Maintain-Guidance.md to describe folder-derived navigation.
• Keep the shared theme, plugins, Markdown extensions, assets, and repository-specific metadata unchanged unless the navigation change requires it.
• Run the documentation index check, link checks, Pester suite, and Zensical build.
• Verify the rendered top-level and nested navigation is complete and alphabetically ordered.

References

• 🚀 [Feature]: Module repositories share one auditable baseline PSModule/Process-PSModule#525
• 🪲 [Fix]: New modules inherit the current repository baseline PSModule/Template-PSModule#53
• Zensical navigation