Rename any _all.scss files to _index.scss so they work as index files

[36degrees]

What

Rename the all files that act as entrypoints to each of our Sass layers.

Retain the all files as aliases, but add deprecation warnings when importing via them so we can remove those files in the next major release.

Update any documentation referring to the all files to reference the new index files.

Why

Use index as the entrypoint for each layer as it:

• matches how we’re structuring the Sass for components
• allows users to omit ‘index’ from the filename where their Sass compiler supports index files
• is a small stepping stone towards moving to the Sass module system

Who needs to work on this

Developers

Who needs to review this

Developers, tech writer

Done when

• We have index files for each of our Sass layer entrypoints
• We've updated any internal imports to use the new index files
• We've retained the existing all files as aliases for the index files, and they output warnings when imported
• We've updated any documentation that refers to the all files to reference the index files instead.