Add Subscription Option for Wiki Pages

Description

Currently, users can subscribe to work items to receive notifications about changes and updates. However, this functionality is not available for Wiki pages. We need to implement a subscription option for Wiki pages that allows users to stay informed about changes without manually checking pages or relying on @ mentions in comments.

Current Behavior

• Users can subscribe to work items (issues, merge requests, epics) to receive notifications about updates
• No equivalent subscription option exists for Wiki pages
• Users must either manually check Wiki pages for updates or rely on @ mentions in comments

Expected Behavior

• Users can subscribe to individual Wiki pages
• Users receive notifications when subscribed Wiki pages are modified
• Subscription status is clearly visible and can be toggled easily

Business Value

Adding subscription capabilities to Wiki pages will provide significant value to customers by:

1. Improving knowledge sharing - Team members can stay informed about documentation changes without additional effort
2. Reducing information silos - Critical updates to documentation won't be missed by relevant stakeholders
3. Enhancing collaboration - Makes Wiki pages more interactive and encourages more active participation in documentation
4. Supporting compliance requirements - Helps teams that need to track and verify when documentation is updated
5. Increasing Wiki adoption - Makes Wikis more useful as living documentation by ensuring updates reach interested parties
6. Providing consistency - Creates a uniform experience across the platform (work items and Wiki pages)
7. Supporting distributed teams - Particularly valuable for teams working across different time zones who need asynchronous updates