Add "Learn Git Branching" (ja: interactive-tutorials)

[SuzukiKatsuma]

What does this PR do?

Add resource

For resources

Description

Add "Learn Git Branching"

Why is this valuable (or not)?

This is a website where you can learn about Git interactively.

How do we know it's really free?

It's OSS, released under the MIT licence.

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

Interactive Programming Resources

Checklist:

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

Follow-up

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

[github-actions[bot]]

Linter failed, fix the error(s):

  free-programming-books/more/free-programming-interactive-tutorials-ja.md
    3:1-7:20  warning  Incorrect number of blank lines between last section and next heading  blank-lines-1-0-2  remark-lint
  11:1-11:69  warning  Incorrect number of blank lines between last section and next heading  blank-lines-1-0-2  remark-lint