Redesign homepage docs

- New Bootstrap purple navbar
- Redesigned masthead
- Rewrote and redesigned homepage content
- Replace Copy text with icons like Bootstrap Icons site across all ClipboardJS instances
- Fixed padding issues in site footer
- Match homepage button styles to examples page, use gap instead of tons of responsive margin utils

scss/_utilities.scss

@@ -462,6 +462,7 @@ $utilities: map-merge(
         lighter: $font-weight-lighter,
         normal: $font-weight-normal,
         bold: $font-weight-bold,
+        semibold: $font-weight-semibold,
         bolder: $font-weight-bolder
       )
     ),
@@ -589,6 +590,8 @@ $utilities: map-merge(
         1: $border-radius-sm,
         2: $border-radius,
         3: $border-radius-lg,
+        4: $border-radius-xl,
+        5: $border-radius-2xl,
         circle: 50%,
         pill: $border-radius-pill
       )