New position utils, split docs

- Renames .pos-f-t to .fixed-top, adds .fixed-bottom and .sticky-top
- Updates utilities Sass to be split across more files (one for position, sizing, and spacing)

scss/utilities/_sizing.scss

@@ -0,0 +1,10 @@
+// Width and height
+
+@each $prop, $abbrev in (width: w, height: h) {
+  @each $size, $length in $sizes {
+    .#{$abbrev}-#{$size} { #{$prop}: $length !important; }
+  }
+}
+
+.mw-100 { max-width: 100% !important; }
+.mh-100 { max-height: 100% !important; }