[birt-dev] Checkin: Add detail description for new multi page contributo

["Qiangsheng Wang" <qwang@xxxxxxxxxxx>]

- Summary: Add detail description for new multi page contributor extension pint.
- Bugzilla Bug (s) Resolved: 128609

- Description:

Add detail description for new multi page contributor extension pint. This extension point decouple the core ui feature with other extended feature or apis, like preview editor and report engine api.

-          Tests Description : manual

- Files Edited:

-          org.eclipse.birt.report.designer.ui.editors\schema\multiPageEditorContributor.exsd

- Files Added:

- Notes to Build Team:
None

- Notes to Developers:
None

- Notes to QA:
None

- Notes to Documentation:

--Qiangsheng Wang