Add user guide for Demon chatbot

Add a comprehensive user guide to assist new users in navigating
through the Demon's features and functionalities.
The guide includes:

- Installation steps
- Basic usage and commands

This documentation aims to enhance the user experience by providing
clear instructions and support for our chatbot.

Add directory for saving Demon.txt file

Add duplicate task check functionality

This commit introduces new feature to check for duplicates when adding a new task. This feature aims to prevent the addition of tasks that are exactly identical in description and, where applicable, date and time for Deadline and Event tasks.

Key changes:
- Implemented `checkDuplicate` method to validate uniqueness across ToDo, Deadline, and Event tasks based on their descriptions and date/time.
- Refactored Deadline, Event, and Task class to accommodate the addition of the new feature.
- Introduced exception handling for duplicate tasks to notify users of attempted duplications and prevent task addition.

This functionality is crucial for maintaining an organized and efficient task list, ensuring that users do not inadvertently clutter their task list with duplicate entries.

Add assertions to validate internal assumptions

This commit introduces a series of assert statements across various methods to ensure that internal assumptions about the state and integrity of data are explicitly checked during development.

- Assert non-null task lists in task management methods to prevent NullPointerExceptions.

These assertions are intended for use in development and testing phases to catch logical errors and assumptions violations early. They are disabled in production to avoid impacting performance.

Refactor methods for improved abstraction

This commit restructures the methods in Command class to align with the Single Level of Abstraction Principle (SLAP). The main changes include:

- Abstracting all methods such that it is more readable.

Benefits Achieved:
- Enhanced readability as the methods has the same level of abstractions.
- Easier debugging as functionalities are isolated into their own methods.

Add assertions to validate internal assumptions

This commit introduces a series of assert statements across various methods to ensure that internal assumptions about the state and integrity of data are explicitly checked during development.

- Assert non-null task lists in task management methods to prevent NullPointerExceptions.

These assertions are intended for use in development and testing phases to catch logical errors and assumptions violations early. They are disabled in production to avoid impacting performance.

Merge branch 'Level-10'

Merge branch 'Level-9'

# Conflicts:
#	src/main/java/demon/Commands.java

Merge branch 'A-JavaDoc'

Merge branch 'A-CodingStandard'