move all the CSS variable to a single folder

[12rambau]

description

The variables were split between assets and static making it difficult to track them.
In this PR I decided to follow the structure used in Furo: https://github.com/pradyunsg/furo/tree/main/src/furo/assets/styles/variables with a single variables folder and split the variables in small sementic files.

There are then called in index.scss

side modifications

• I edited the documentation to make it point to the correct files
• I changed sidebar variables to respect ng-sphinx structure

question

I have little understanding of the webpack builder so I left the theme.css file in static, it's just reduced to 1 single line. I think webpack.conf.js could be updated to directly import this unique file. If someone knows, please let me know in the review.

related to

Fix #676

[choldgraf]

This seems like a reasonable change to me! I think that we might need to keep the theme.css just to have that import statement in there, I'm not sure how else to pass a "regular CSS import statement" into an SCSS file without it being compiled.