Remove navigation.indexes from Zensical features (#35)

## Summary

Removes `navigation.indexes` from the Zensical theme features in
`.github/zensical.toml`.

## What changed

- Dropped `navigation.indexes` from the `[project.theme]` features list

## Why

`navigation.indexes` creates clickable section index headers in the
left-side navigation menu of the generated Zensical docs site. Removing
it eliminates that visual header, resulting in a cleaner left-side
navigation without the section-level heading element.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: MariusStorhaug

.github/zensical.toml

@@ -11,7 +11,6 @@ favicon = "Assets/icon.png"
 features = [
     "navigation.instant",
     "navigation.instant.progress",
-    "navigation.indexes",
     "navigation.top",
     "navigation.tracking",
     "navigation.expand",