Add GitLab Unix shell scripting style guide

Description of the proposal

Across GitLab, we use pretty much of Unix shell scripting (sh, bash and maybe something else). But we don't have an appropriate style guide yet. This is a proposal to:

• create a blueprint of such a style guide

• discuss and contribute to the contents

• adopt it and add to the documentation

• would be great to also find and adopt linters to check the compliance in future

• Mention the proposal in the next backend weekly call and the #backend channel to encourage contribution

• Proceed with the proposal once 50% of the maintainers have weighed in, and 80% of the votes are 👍

• Once approved, mention it again in the next backend weekly call and the #backend channel

/cc @gitlab-org/maintainers/rails-backend

Good examples

• https://google.github.io/styleguide/shell.xml
• more?