[NO ISSUE] User-defined Function Documentation update - user model changes: no - storage format changes: no - interface changes: no
Details: 1. Updated the UDF documentation to be consisten with current master. 2. Cleaned default UDF package to remove useless UDFs. 3. Added the example in documentation as a test case for IT. 4. Reorganized the documentation to keep up with the new structure. 5. Minor changes to other documentation pages to keep style consistent.