docs(VParallax): add welcome section example to docs

[mohssineAboutaj]

Description

Add a new example in the Parallax docs page

Demo

Here is the demo/live preview

Markup:

<template>
  <v-parallax src="https://cdn.vuetifyjs.com/images/backgrounds/vbanner.jpg">
    <v-container class="fill-height">
      <v-row
        class="justify-center align-center flex-column-reverse flex-md-row"
      >
        <v-col cols="12" md="6">
          <h1 class="text-h1 mb-8">John Doe</h1>
          <h3 class="text-h3 mb-8 font-weight-thin">Web Developer</h3>
          <v-btn color="primary" class="elevation-4 rounded-xl mb-4">
            Contact Me
          </v-btn>
        </v-col>
        <v-col cols="12" md="6" class="text-center">
          <v-avatar :size="300" class="elevation-12 mx-auto mb-8">
            <v-img src="https://randomuser.me/api/portraits/men/78.jpg"></v-img>
          </v-avatar>
        </v-col>
      </v-row>
    </v-container>
  </v-parallax>
</template>

fixed