[UPDATED]feat: add activity selection problem

[chhavibansal]

Description of Change

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• 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:

[realstealthninja]

Please document everything according to https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/CONTRIBUTING.md

[chhavibansal]

@realstealthninja @Panquesito7 can u please review

[chhavibansal]

@realstealthninja can u please review

[realstealthninja]

This file is still not documented properly!! please check the contributing guidelines and document accordingly. thanks!

[github-actions]

This pull request has been automatically marked as abandoned because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions]

Please ping one of the maintainers once you commit the changes requested or make improvements on the code. If this is not the case and you need some help, feel free to ask for help in our Gitter channel or our Discord server. Thank you for your contributions!