docs: add workspace improvements report for v3.0.0

[tkykenmt]

Summary

This PR adds documentation for the Workspace Improvements bugfix in OpenSearch Dashboards v3.0.0.

Reports Created

• Release report: docs/releases/v3.0.0/features/opensearch-dashboards/workspace-improvements.md
• Feature report: docs/features/opensearch-dashboards/workspace.md (new)

Key Changes in v3.0.0

• Saved Objects Isolation Fix (PR #9420): The _find API now correctly returns only saved objects within the current workspace context
• Recent Items Error Filtering (PR #9346): Deleted or inaccessible items are filtered out from the recent items list
• Stale Workspace Error Handling (PR #9478): Users are redirected to an error page instead of getting stuck in a loading state when accessing a stale workspace

Resources Used

• PR: #9420, #9346, #9478
• Docs: https://docs.opensearch.org/3.0/dashboards/workspace/workspace/

Closes #282