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/_position.scss

@@ -0,0 +1,23 @@
+// Positioning
+
+.fixed-top {
+  position: fixed;
+  top: 0;
+  right: 0;
+  left: 0;
+  z-index: $zindex-fixed;
+}
+
+.fixed-bottom {
+  position: fixed;
+  right: 0;
+  bottom: 0;
+  left: 0;
+  z-index: $zindex-fixed;
+}
+
+.sticky-top {
+  position: sticky;
+  top: 0;
+  z-index: $zindex-sticky;
+}