Adds shared SettingsSection helper component

What does this MR do and why?

Adds reusable SettingsSection component

This lets us migrate all those loose .settings-section integrations in various settings pages.

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Migrates Admin settings: Appearance as an example page, see http://gdk.test:3000/admin/application_settings/appearance

Usage

HAML: http://gdk.test:3000/rails/lookbook/inspect/layouts/settings_section/default

= render ::Layouts::SettingsSectionComponent.new(_('Settings section heading')) do |c|
  - c.with_description do
    = _('Settings section description')
  - c.with_body do
    = _('Settings section content')

Vue: http://localhost:9002/?path=/story/vue-shared-settings-settings-section--default

<script>
  import SettingsSection from '~/vue_shared/components/settings_section.vue';
</script>

<template>
  <settings-section heading="Settings section heading">
    <template #description>
      <!-- Description slot -->
    </template>

    <!-- Body slot -->
  </settings-section>
</template>

Related to #457239 (closed)