Add a section for strings

I wish there was a section on strings. How to index them, some formatting tricks, etc.

Look at the existing cheat sheet structure in the repository to see how sections are organized. The new section should cover string indexing, common formatting methods like fmt.Sprintf, and perhaps the strings package. Check for any tests or examples to ensure consistency. The work is done when the new strings section is added and follows the project's format.

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