Add option to prevent auto-collapse of expanded menu items

[AntonShuvaev]

Feature Description
When expanding a menu item in the sidebar, allow multiple sections to stay expanded instead of auto-collapsing previously expanded items.

Problem/Solution
Current behavior: When clicking to expand a menu item in the sidebar, any previously expanded items automatically collapse, creating a single-expansion behavior. This is not a common pattern in documentation sites - for example, Nextra keep items expanded.
Proposed solution: Add configuration option to control collapse behavior:

params:
 sidebar:
   autoCollapse: false