Add Japanese course and book (Scratch, C++)

[SuzukiKatsuma]

What does this PR do?

Add resources

For resources

Description

Add links to Scratch and C++ books.

Why is this valuable (or not)?

• Scratch for CS First でプログラミングをはじめよう
  This programming material is published by Google.

• C++入門 AtCoder Programming Guide for beginners (APG4b)
  It is published by AtCoder, a company that operates programming contest site.
  Learn the fundamentals of general-purpose programming.

How do we know it's really free?

• Scratch for CS First でプログラミングをはじめよう
  This website is published under the Creative Commons License BY-SA 4.0.

• C++入門 AtCoder Programming Guide for beginners (APG4b)
  It is offered free of charge by AtCoder as an introduction to competitive programming.

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

• Scratch for CS First でプログラミングをはじめよう is course.
• C++入門 AtCoder Programming Guide for beginners (APG4b) is a book.

Checklist:

• Read our contributing guidelines
• 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)

Follow-up

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