* Clarify Sass import and customize docs for how to modify variable defaulst
* Add an npm starter project callout to a few pages
* Update callout-info-npm-starter.md
Co-authored-by: XhmikosR <xhmikosr@gmail.com>
* Add Sass section to Getting Started > Build Tools, mention Sass implementation used, mention current precision of 10 due to Dart Sass, and mention recommended minimum precision (value of 6 used in BS v4). Remove outdated Sass precision from Customize > Sass docs page.
* Remove more mentions of deprecated Sass implementations. Add links to Sass compiler mentions so we can keep all pertinent Sass compiler information in just one location instead of spread through the docs, which makes it hard to find the details and/or keep them up-to-date.
Co-authored-by: XhmikosR <xhmikosr@gmail.com>
