Add dropdown menus to horizontal navbar

[svekars]

Adds dropdown menus to horizontal navbar items that have child pages. When hovering over these items on desktop, a dropdown displays (from the underlying toctree) the sub-pages for quick navigation.

• Custom navbar-nav.html template that generates hierarchical navigation from toctree structure
• Python functions in __init__.py to extract toctree children using toctree_includes
• CSS styles in _navbar_dropdown.scss for hover-based dropdowns

Features:

• Desktop: Hover to show dropdown with child pages, click parent link to navigate
• Mobile: Dropdowns hidden, parent link navigates to page directly
• Scrollable dropdown for long lists (max-height: 70vh)
• Respects header_links_before_dropdown config for "More" button
• Pure CSS/Jinja implementation, no additional JavaScript
• Enabled by default, can be disabled by setting html_theme_options: "enable_navbar_dropdowns": False

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