Add a section on how we use SASS

[alexkilgour]

• Use of dart-sass - @use/@forward etc
• calc() vs sass:math

[sturobson]

As a long time owner (9 years I think) of SassnotSASS.com -- I wonder if we can change any SASS to Sass at the same time :)

Happy to do a separate PR with a find and replace of sorts :)