Carts on the website should support multiple authors

[aduros]

Some games are authored by a team of authors, but the cart manual format and web site is only set up to display a single author. We should support multiple authors so that everybody gets credit for working on a game!

Here's a proposal: change the markdown frontmatter on cart manuals to look like this (simple example):

author: Bruno <aduros>

and an example with multiple authors:

author: Alice <alice_on_github>, Bob <https://twitter.com/bob>, Chris <chris@gmail.com>

Some notes about this:

• author would be comma separated to support multiple authors
• Use a format similar to email (Name <contact>) to attach any contact info to the name, whether it be a GitHub account name, email address, or website URL.
• No longer need a separate github frontmatter field.

For this proposal here are the places that would need updating:

1. wasm4/site/docusaurus.config.js

   Lines 179 to 187 in 4d59146

   	const author = markdown.frontMatter.author;
   	if (!author) {
   	throw new Error("Missing author");
   	}
   	const github = markdown.frontMatter.github;
   	if (!github) {
   	throw new Error("Missing github");
   	}

   This parses all the cart manual frontmatter and makes it available to React.

2. wasm4/site/src/components/PlayCart.js

   Lines 63 to 76 in bb0b187

   	<div className="avatar">
   	<a
   	className="avatar__photo-link avatar__photo avatar__photo--lg"
   	href={`https://github.com/${cart.github}`} target="_blank"
   	>
   	<img src={`https://github.com/${cart.github}.png?size=128`} />
   	</a>
   	<div className="avatar__intro">
   	<div className="avatar__name"><a href={`https://github.com/${cart.github}`} target="_blank">{cart.author}</a></div>
   	<small className="avatar__subtitle">
   	{dateString}
   	</small>
   	</div>
   	</div>

   UI for the individual cart play page.

3. wasm4/site/src/components/PlayButton.js

   Lines 19 to 22 in 177effb

   	<div className="card__footer">
   	<div className="avatar__name">{title}</div>
   	<small className="avatar__subtitle">{author}</small>
   	</div>

   UI for the buttons that we show on the carts listing.

4. wasm4/scripts/upload-internetarchive

   Line 33 in 650f430

   "creator": cart["author"],

   The IA uploader probably needs to be updated to handle multiple creators and the new author format.
5. Go through the existing carts and update them to the new author format, including handling carts like this one with multiple authors:

   wasm4/site/static/carts/pega-pega-the-game.md

   Lines 2 to 3 in dc634da

   	author: Ana Paula Crippa, Maria Eduarda Policante
   	github: anapaulacrippa, poli404

6. Update the documentation on the website about how to use the new author format (with examples):

   wasm4/site/docs/guides/distribution.md

   Lines 80 to 81 in 12d79e0

   	author: (Your name or username)
   	github: (Your github username, used to show a profile pic)