Add Sass docs (variables, mixins, and loops) to most pages (#32747)

* WIP: Mention variables, mixins, and loops in docs

* Add Sass sections to component pages

* add sass docs for forms and content

* Update buttons.md

* Remove empty mixins sections

* Massive update to utilities and some consistency changes

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

site/content/docs/5.0/forms/checks-radios.md

@@ -269,3 +269,9 @@ Different variants of `.btn`, such at the various outlined styles, are supported
 <input type="radio" class="btn-check" name="options-outlined" id="danger-outlined" autocomplete="off">
 <label class="btn btn-outline-danger" for="danger-outlined">Danger radio</label>
 {{< /example >}}
+
+## Sass
+
+### Variables
+
+{{< scss-docs name="form-check-variables" file="scss/_variables.scss" >}}