Migrate styles from deprecated file and remove all comments

- Migrated guide page styles (Best Practices, Clear Goals, Establish Trust, Write for Action)
- Created _guides.scss component with all missing styles
- Fixed SMS message bubble dimensions to match original design (464px, 5px border-radius)
- Reduced header divider height between Settings | Sign out
- Removed duplicate placeholder styles from _sms-message.scss
- Created design token for placeholder highlight color (#face00)
- Removed all unnecessary comments from SCSS files
- Migrated new fieldset and radio background styles from main branch

app/assets/sass/uswds/overrides/_forms.scss

@@ -16,3 +16,11 @@
     max-width: units('mobile');
   }
 }
+
+.usa-fieldset {
+  background-color: transparent;
+}
+
+.usa-radio {
+  background-color: transparent;
+}