[docs] Add collapsible sidebar support - #4551

Purpose

This pull request adds support for a collapsible documentation sidebar on the Apache Fluss website. Users can collapse the left sidebar to expand the main reading area and easily expand it back. This idea was inspired from apache paimon.

Brief change log

• Configured themeConfig.docs.sidebar.hideable: true in website/docusaurus.config.ts to enable collapsible sidebar support with state persistence.
• Scoped blog button rules (.button.button--secondary and ::after arrow) in website/src/css/custom.css to prevent unintended styling and pseudo-elements on the sidebar collapse button.
• Added custom styles for the bottom sidebar collapse bar (<<) and left-edge expand button (>>) to ensure seamless theming and smooth hover transitions in both light and dark modes.
• Verified that desktop and mobile/tablet drawer navigation work cleanly without layout regressions.

Tests

• Manually tested desktop sidebar collapse and expand behavior in both light and dark modes.
• Manually verified responsive behavior on mobile/tablet viewports.

here are some looks

API and Format

Does not affect API or storage format.

Documentation

Does not introduce changes to documentation content.