[Documentation] Update contents of help files

[ZiXian92]

Main help page can act as just a directory to help pages of other commands by listing the commands to access those help pages. Copying straight from user guide makes it very wordy. I won't want to read through a wall of text when I just want to quickly find out how to use the command. Will make some changes to make them concise once I am done with the default sorting mechanism.

[nhan-ng]

Agree!

Main help page will explain about:

• The general structure of the command and how Dynamiz will parse the command.
• The common aliases used for CommandType and OptionType. (When the customisation is implemented, it can also be mentioned here and explained in details in a separate file).
• The displayed output. May also mention about where the tasks are saved and the the archiving of the completed task.
• A general list of other commands: help add, help etc.

Individual command help page will have a structure similar to the following:

• Generic structure of the command.
• Give how each different structure behaves (applicable only to commands having multiple structures, namely, add, list, search, update)

Reference style: How to add a task in Google Tasks (https://support.google.com/calendar/answer/144246?hl=en)

[nhan-ng]

Contents updated. @ZiXian92 @XYLau @Vio8023

Please check the contents and let the team know if there is anything to be changed/added/deleted. Also, it's a good way to get familiar with out product.

PS: Help me check the Grammar as well because I wrote the Help files with one eye open.