cleanup indents and git attributes for documented contents

[fmigneault]

• Adds .gitattributes to mark content of interest (https://github.com/github-linguist/linguist/blob/main/docs/overrides.md#detectable)
  Without this, the repository currently reports some odd statistics (as shown below) because those files are often used by CI/configs, and therefore ignored by default.

  

• unify openapi indents
  Because files where edited/imported in various ways, parsers have trouble dealing with the mixture of indents. Use the typical defaults for them (2-spaced indent style, and what most of the files already employed) to make it consistent.

• add .editorconfig to hint IDEs/text-editors supporting it to respect the desired indents