Update User Guide with new grading feature

[yongqqqq]

Description

This PR updates the User Guide to document the new GradeCommand feature.

Main Changes:

• Added instruction for GradeCommand, inclusive of examples.

Additional Changes:

• Refactoring to ensure code consistency
• Fix codestyle issues

Testing:

• All existing and new test cases pass
• This verifies that the updated module field is being parsed and handled correctly.

TODO:

• Update Ui for displaying module grades.

closes #75

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
...ava/seedu/address/logic/commands/GradeCommand.java	91.66% <ø> (ø)	11.00 <0.00> (ø)