Add intro.md

[google-labs-jules]

Analyzed the repository structure and content to create an intro.md file.
This file provides an overview of the repository, including its purpose (TIL), major categories (Java, Spring, Database, etc.), and highlights key contents.
The content is written in Korean, matching the user's request and the repository's language.

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PR created automatically by Jules for task 14458474420246173742 started by @CUCU7103

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

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I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

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