Document how to use Gitpod, GitLab's Web IDE, or GitHub's Code Spaces

[umarcor]

• Ready-to-use IDE based on containers and VSIX #45
• Gitpod
  • gitpod.io
  • .gitpod.yml, .gitpod.Dockerfile
• Gitlab Web IDE
  • docs.gitlab.com/ee/user/project/web_ide
• GitHub Code Spaces
  • github.com/features/codespaces
  • .devcontainer