Added explanation for [All Combinations Of Size K}

[Success1308]

Description

This pull request adds a detailed explanation for the problem "All Combinations Of Size K" in the Backtracking section. The explanation includes the problem statement, approach, time complexity, space complexity, applications, step-by-step instructions, and examples to ensure clarity and accessibility.

Checklist

• A description of the changes in this PR is mentioned above.
• All the new and existing checks pass.
• No plagiarized, duplicated, or repetitive documentation that has been directly copied from another source.
• If it's a new explanation, it contains solid, understandable, and accessible information.
• I have read the whole contributing guidelines and agree to the Code of Conduct.

Note to reviewers

This explanation aims to provide a comprehensive guide to solving the "All Combinations Of Size K" problem using the backtracking approach. It is beginner-friendly, well-structured, and aligned with the repository's standards.

[Success1308]

Reviewers,
is there any particular time line that we should expect for this ?

[github-actions]

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[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. Thank you for your contributions!