[FR] Option to hide sub-pages from left sidebar navigation tree

### Description

Currently, whenever a sub-page is created within a main page (e.g., using /page or nesting), AppFlowy automatically expands and shows every nested page in the left sidebar as a tree structure.

For users who create many sub-pages within a topic/folder, the sidebar quickly becomes overly cluttered and visually overwhelming. While collapse is available, it doesn't solve the core UX issue for those who prefer a clean Notion-like navigation bar where sub-pages only exist within the parent page's content canvas, not cluttering the main sidebar tree.

Describe the solution you'd like
I would like to request an option/setting to hide sub-pages (nested pages) from appearing in the left sidebar, or allow a "flat sidebar" mode.

Ideally:

A setting toggle: "Hide sub-pages in sidebar" (so only top-level pages/folders are shown in the sidebar).

Or an option per page/folder: "Hide child pages from sidebar tree".

Describe alternatives you've considered
Currently, the only workaround to keep the sidebar clean is using Grid - Inline databases to hold sub-pages. While functional, it is not as intuitive or seamless as natively creating inline pages without clogging the sidebar tree.

Additional context
Providing flexibility between a deep folder-tree sidebar (Obsidian/Logseq style) and a clean decoupled sidebar (Notion style) would greatly improve the writing and organizational experience for many users.

### Impact

Users who create structured notes with multiple sub-pages (e.g., project documentation, knowledge bases, meeting logs) and prefer a clean, uncluttered sidebar navigation. It also helps users transitioning from Notion who are accustomed to separating the main canvas view from the sidebar tree.

### Additional Context

Currently, using Grid - Inline databases is the only workaround to keep sub-pages hidden from the sidebar, but it feels less seamless than natively toggling or hiding sub-page visibility in the sidebar. Allowing an option to toggle sub-page visibility in the sidebar would greatly balance the needs of users who like deep folder trees and those who prefer minimal navigation.