refactor: implement group-based pagination in DocsNavigation

[Yeom-JinHo]

Description

This PR refactors the DocsNavigation pagination logic to enable group-based navigation.
Previously, navigation followed the raw order of config/docs.ts, resulting in an inconsistent sequence across unrelated components.
Now, navigation prioritizes movement within the same group, and once a group ends, it automatically continues to the next group.

Changes

• Group-Based Pagination: Added logic to handle prev/next transitions within each group, then proceed to the next or previous group when reaching boundaries.
• Consistent Order: Navigation is no longer based on the raw array order of config/docs.ts; it now respects logical grouping.
• Simplified Logic: Implemented a reduce + flat structure for cleaner and more maintainable iteration.
• Type Safety: Explicitly defined types (Record<string, DocsNavigationCategory[]>) to remove @ts-ignore and ensure stable autocomplete.

Motivation

To eliminate inconsistent navigation caused by the arbitrary order of config/docs.ts and align pagination with the actual component grouping.
This makes the browsing experience more structured — allowing users to explore related components together before moving on to the next group.