docs: add missing GitHub issue templates and reorder

Added three missing enhancement issue templates:
- #6 Extract Pages method (simpler alternative to split_pdf)
- #10 Convert to Office Formats (DOCX, XLSX, PPTX export)
- #13 Batch Processing (client-side bulk operations)

Reordered existing templates to maintain logical sequence.
All 13 enhancements now have corresponding issue templates.

Author: jdrhyne