[Concept Entry] Kotlin: Adding string.md in kotlin folder

[dakshdeepHERE]

Reason/inspiration (optional)

I would like a new concept entry for Kotlin about strings.

The entry will go in a new file in docs/content/kotlin/concepts/strings/string.md

The entry will have:

• Have a section on string description.
• Have a syntax pseudo block for strings.
• Have an example code block for string.

Entry Type (select all that apply)

New Concept Entry for Existing Topic

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