Refactor SCSS structure

[choldgraf]

This refactors our SCSS so that it follows a more standard structure that aligns with sphinx-basic-ng, sphinx-book-theme, furo, etc. I don't believe it changes the end-result of anything, it only updates the location of SCSS files so that they are more modular, extendable, etc.

ref: #577

[choldgraf]

pinging @damianavila on this one - this will break quickly when people make PRs, so if possible it would be great to get this one merged soon. I'm happy to iterate on some feedback though!

[12rambau]

@choldgraf I really like how you split everything as I love small files with meaningful names, that's way easier o search in an IDE.

I don't know much about the inspiration you're quoting in #577 but I assume you respected some kind of convention and I think it works very well (special thanks for sidebar-primary and sidebar-secondary).

I have 2 questions though:

• why the underscores? I use "_" to differentiate private from public stuff in Python but here everything is the same so why bother?
• any reason why you created empty files (_toc-inpage, _tables, _variables)?

I don't feel legit to give any formal approval but I like the new structure!

[damianavila]

@choldgraf, some comments:

1. This looks great to me (I left some tiny comments). I love the new structure!!
2. This PR already has some merge conflicts because of some recent merges. Let's stop merging new stuff until we land this one.
3. Let's merge this one in the next 24-48 hs.

[choldgraf]

OK I believe that I've cleaned up this PR and it's ready to merge - conflicts are gone and I've added back in the missing files that you mentioned, thanks for catching that @12rambau !

[damianavila]

LGTM and merging right away so it does not get old! 😉