update Hugo and deploy with GitHub Actions

[ryotarotemma]

Changes

• Fix removed variables to work with newer Hugo
  • Replace .Hugo.Generator wth hugo.Generator (FYI: https://qiita.com/ikemo/items/5ad35021da46db47bb34)
  • Replace $.Page.Dir with $.File.Dir
  • Fix alias to follow-up new version behavior.
• Deploy with GitHub Actions not to host git managed static files. Hosting git managed static files leads unexpected behavior, because building static file with hugo command does not remove files, just create or update files.
  • Remove docs dir.
  • Add GitHub Actions config. By deciding base-url in GitHub Actions, remove it from hugo config(so it works better in local server).
  • Add .gitignore file to prevent committing command generated files.

Tasks left

• Change Github Pages deploying option in the repository seetting.
  • We should test before merging.

[ryotarotemma]

@blue0513 @scova0731 レビューお願いします〜 docsディレクトリのアコーディオンを閉じると、見るべきファイルがわかりやすくなると思います