Add key to stylesheet links in Head.js

[hshoff]

Motivation

I was adding a google font through siteConfig.stylesheets and noticed the following error:

Starting Docusaurus server on port 3000...
Open http://localhost:3000/
Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of `Head`. See https://fb.me/react-warning-keys for more information.
    in link (created by Head)
    in Head (created by Site)
    in html (created by Site)
    in Site (created by DocsLayout)
    in DocsLayout

This fixes that by adding the key prop. Following the same style as used for siteConfig.scripts

Test Plan

• ran npm run prettier
• npm run test pass
• verified with npm link that error message was gone with this patch applied

[JoelMarcey]

@hshoff Thank you very much. I have been wanting to fix this for a while, and just kept putting it off.

I think this is a reasonable way to do the key.

cc @hramos Do you see a problem with this? I am going to merge it, but we can change it if you think it is a problem.