Remove duplicate sidebar header in docs nav

[Marius Storhaug (MariusStorhaug)]

Summary

• remove the
  avigation.indexes feature from .github/zensical.toml to drop the redundant left sidebar header
• align palette and theme settings with PSModule/docs (black/light-blue palette, material theme icons, and matching navigation/search feature toggles)
• align TOC permalink style with PSModule/docs

Why

The docs navigation had a duplicated section header in the left menu, and this also brings Sodium docs styling/settings closer to the PSModule docs baseline.

Testing

• docs config change only; no runtime/module code changes

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MARKDOWN	Pass ✅
NATURAL_LANGUAGE	Pass ✅
POWERSHELL	Pass ✅
PRE_COMMIT	Pass ✅
SPELL_CODESPELL	Pass ✅
TRIVY	Pass ✅
YAML	Pass ✅

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No Significant Changes Detected

This PR does not contain changes to files that would trigger a new release:

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[github-actions]

No Significant Changes Detected

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^PSModule/	Matches files where path matches this pattern

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No Significant Changes Detected

This PR does not contain changes to files that would trigger a new release:

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^README\.md$	Matches files where path matches this pattern
^PSModule/	Matches files where path matches this pattern

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[github-actions]

No Significant Changes Detected

This PR does not contain changes to files that would trigger a new release:

Pattern	Description
^src/	Matches files where path matches this pattern
^README\.md$	Matches files where path matches this pattern
^PSModule/	Matches files where path matches this pattern

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