✨feat(quick-navigation): add Table of Contents button to quick navigation buttons

[welpo]

Description

This PR introduces a new Table of Contents (ToC) button to the quick navigation section. When clicked, it shows a floating ToC, facilitating navigation.

Features

• Quick navigation integration: The ToC button is integrated into the existing quick navigation buttons section.
• ToC visibility: The ToC button will only appear for articles containing headers, ensuring a clean UI for content without a ToC.
• Overlay handling: An overlay functionality is implemented, allowing users to interact with the ToC intuitively. Clicking on the overlay will close the ToC.

Files Changed

• Modified the Tera template to add the ToC button.
• Moved quick navigation buttons styling from _misc.scss to _quick_navigation_buttons.scss.
• Updated SCSS files for styling the ToC button and overlay.

Example