First go at adding authors.

[simonprickett]

This PR implements the first step for for adding authors by allowing author bylines to be configured per page. Closes #103

You add an authors array in your front matter like so:

---
id: index-hacktoberfest
title: Hacktoberfest 2021 at Redis
sidebar_label: Hacktoberfest 2021
slug: /hacktoberfest/
authors: [suze,simon]
---

Then you add details of each author to docusaurus.config.js each being an object like this:

  ...
  customFields: {
    authors: {
      simon: {
        name: 'Simon Prickett',
        link: 'https://twitter.com/simon_prickett',
        title: 'Manager, Developer Advocacy',
        image: 'profile_pic_simon_prickett.jpg'
      },
      suze: {
        name: 'Suze Shardlow',
        link: 'https://twitter.com/SuzeShardlow',
        title: 'Developer Community Manager'
      }
    }
  },
  ...

If no image is present for an author, a default fallback silhouette image will be used. link can be any link you want for now... long term the idea is we'd link to an author page and have a bio and socials about each author there.

I then overrode the DocItem component from the theme, and added extra JSX to render these values in a really basic way to prove it works... we can style these more in a future PR and also create a page per author with their bio and articles they have written.

Here's an example of what it looks like. The Hacktoberfest page has authors set on it as an example. If none are set, the component doesn't render anything and the page won't have the author information.

I've also updated the README to cover how to add an author.

[ajeetraina]

LGTM