[translation] [Japanese] translate some pages

[ysaito8015]

I'll do these...

Organise Pages

delete a page

• [x] ./_index.ja.md

Translate Pages

• [ ] ./content/how-to/troubleshoot-gpu-issues.ja.md
• [ ] ./content/reference/cuda.ja.md
• [ ] ./content/reference/vm/gomachine.ja.md
• [ ] ./content/tutorials/mnist-cuda.ja.md

Update Contents and Translation

• [x] ./content/about/_index.ja.md
• [x] ./content/getting-started/_index.ja.md
• [x] ./content/how-to/autodiff.md