Rewrite Theming docs as Customize section

- Broke up existing theming page into separate pages
- Audited and updated each new docs page (some pages include some TBD sections)
- Update sidenav to reflect changes
- Fix links that pointed to the old Theming page
- Update docs styles to reflect recent changes
- Rewrite some bits in the Migration page

site/assets/scss/_content.scss

@@ -29,6 +29,7 @@
   > .table {
     max-width: 100%;
     margin-bottom: 1.5rem;
+    @include font-size(.875rem);
 
     @include media-breakpoint-down(md) {
       display: block;