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 ?

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