Add Greek (el) C# category and book: "Επίλυση προβλημάτων με τη γλώσσα C#"

[Maria-Papa]

What does this PR do?

Add resource(s)

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Description

A beginner-friendly Greek textbook that teaches programming through problem-solving using the C# language. It covers core concepts like variables, control structures, functions, arrays, file handling, and object-oriented programming, with practical examples aimed at students and self-learners.

Why is this valuable (or not)?

This resource provides a structured introduction to C# programming in Greek, making it highly accessible to native speakers. It covers key topics such as variables, control flow, functions, data structures, file handling, networking, and database connectivity, ideal for learners aiming to build real-world applications.

How do we know it's really free?

The book is published by Kallipos, a government-funded open academic publishing platform in Greece. It is distributed under the Creative Commons BY-NC-SA 4.0 license, which permits free access and non-commercial redistribution with attribution.

For book lists, is it a book? For course lists, is it a course? etc.

It is a book.

Checklist:

• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction).
• Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!

[Ajay-singh1]

This has been added in PR #12268 . You should remove this otherwise LGTM.

[Maria-Papa]

@Ajay-singh1 I'm so sorry.
I removed it!

[Ajay-singh1]

LGTM.