[docs] Migrate SCSS

[sarjona]

The URL of the page on https://docs.moodle.org/dev/

https://docs.moodle.org/dev/CSS_Coding_Style

Is this documentation specific to a Moodle version?

Yes

What location would you suggest in the new docs?

https://moodledev.io/docs/guides/styles

Related with #983

[ziegenberg]

Is this a duplicate of #697?

[sarjona]

Yes, @ziegenberg! You're right! I knew there was some issue but, as I wasn't able to find it, I create it. Thanks!! :-)

I'm going to close this issue (and I also updated #697 description because /docs/guides/styles might fit better :-)