Add initial docs

docs/input/assets/bootstrap/mixins/_grid.scss

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+/// Grid system
+//
+// Generate semantic grid columns with these mixins.
+
+@mixin make-container($gutter: $grid-gutter-width) {
+  width: 100%;
+  padding-right: $gutter / 2;
+  padding-left: $gutter / 2;
+  margin-right: auto;
+  margin-left: auto;
+}
+
+
+// For each breakpoint, define the maximum width of the container in a media query
+@mixin make-container-max-widths($max-widths: $container-max-widths, $breakpoints: $grid-breakpoints) {
+  @each $breakpoint, $container-max-width in $max-widths {
+    @include media-breakpoint-up($breakpoint, $breakpoints) {
+      max-width: $container-max-width;
+    }
+  }
+}
+
+@mixin make-row($gutter: $grid-gutter-width) {
+  display: flex;
+  flex-wrap: wrap;
+  margin-right: -$gutter / 2;
+  margin-left: -$gutter / 2;
+}
+
+@mixin make-col-ready($gutter: $grid-gutter-width) {
+  position: relative;
+  // Prevent columns from becoming too narrow when at smaller grid tiers by
+  // always setting `width: 100%;`. This works because we use `flex` values
+  // later on to override this initial width.
+  width: 100%;
+  padding-right: $gutter / 2;
+  padding-left: $gutter / 2;
+}
+
+@mixin make-col($size, $columns: $grid-columns) {
+  flex: 0 0 percentage($size / $columns);
+  // Add a `max-width` to ensure content within each column does not blow out
+  // the width of the column. Applies to IE10+ and Firefox. Chrome and Safari
+  // do not appear to require this.
+  max-width: percentage($size / $columns);
+}
+
+@mixin make-col-auto() {
+  flex: 0 0 auto;
+  width: auto;
+  max-width: 100%; // Reset earlier grid tiers
+}
+
+@mixin make-col-offset($size, $columns: $grid-columns) {
+  $num: $size / $columns;
+  margin-left: if($num == 0, 0, percentage($num));
+}
+
+// Row columns
+//
+// Specify on a parent element(e.g., .row) to force immediate children into NN
+// numberof columns. Supports wrapping to new lines, but does not do a Masonry
+// style grid.
+@mixin row-cols($count) {
+  & > * {
+    flex: 0 0 100% / $count;
+    max-width: 100% / $count;
+  }
+}