Added subarray material

[ananya0504]

Fixes #118

Short description of what this resolves:

improve: Subarrays

What Changes proposed in this pull request:

• Added material for the subarrays lesson in Java
• Explained what subarrays are.
• Explained how to generate subarrays using an iterative approach
• Explained how to generate subarrays using a recursive approach.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added code examples, test must pass
• Added documentation so that the program is self-explanatory and educational.
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
This lesson contains information about subarrays in Java

[Utkarsh1504]

@ananya0504 Please do the suggested changes, else everything looks nice.

[Utkarsh1504]

Also, remove the first heading as it will be there by default on the site. Thank you

[ananya0504]

Hi @Utkarsh1504
I have improved upon the language.
Added a few extra changes by myself so that the language is easily understandable to the readers.
Also, worked on the formatting as you suggested! Thanks a lot :)

[Utkarsh1504]

Nicely done! 👍🏻 Thank you and keep contributing.