Make font-family as configurable parameter(#220)

[cowlingj]

Motivation

issue: #220

Test Plan

1. Edit siteConfig.js to include fonts object with keys mapped to arrays of font names

fonts: {
  myKey: [
    "Times New Roman",
    "Serif"
  ],
  myOtherKey: [
    "-apple-system",
    "system-ui"
  ]
},

2. add the corresponding keys as font-family variables in a css file in the form $

h1 {
  font-family: $myKey;
}

3. run server

[JoelMarcey]

@cowlingj Thank you very much for this PR to fix the currently hard coded font-family issue.

Did you try, locally, using your new approach in main.css and see if the Docusaurus site, the test site that comes with running docusaurus-init, or any other site would load the same with the proper fonts?

[JoelMarcey]

@cowlingj Hi. Just pinging to see what site you tested this change on? Thanks!

[cowlingj]

I've tested both, first adding custom fonts in the siteConfig.js and custom.css of both, then the docusarus site by building and running with npm, and a site ran with docusarus-init by calling start-server.js directly.

[JoelMarcey]

My Prettier work here is becoming a bit funny. I will get it :)

[JoelMarcey]

@cowlingj I am going to merge this in now. Thank you very much!