Update UG and DG

[ChinSekYi]

resolves:

• [PE-D][Tester C] User Guide Contents Page: Improve Section Titles for Better Navigability https://github.com/AY2425S1-CS2103-F09-3/tp/issues/294
• [PE-D][Tester D] User guide not formatted properly https://github.com/AY2425S1-CS2103-F09-3/tp/issues/283
• Create feature enhancement section in DG https://github.com/AY2425S1-CS2103-F09-3/tp/issues/344
• [PE-D][Tester A] Remark containing N/A to signify N/A can be misinterpreted for name. https://github.com/AY2425S1-CS2103-F09-3/tp/issues/327