[WIP] Design!

[sophshep]

This PR starts to add some design to the guide 🎉

So far, I have:

• Taken out all the legacy CSS and updated with the latest Primer
• Roughed out the homepage layout so it's ready for art
• Styled the typography on the guide pages to match mockups
• Added tables of contents drop downs
  • Note: These have to be added manually in the YAML front matter, since they are part of the title rather than the content - I checked with Parker and there's nothing we can do about this. A bit annoying but not too bad.

Still to do:

• Incorporate @jeejkang's beautiful artwork
• Style the footer
• Fancy blockquote styles
• Vignette styles

Questions/notes for @nayafia & @bkeepers

• Will we have a working search?
• What content would you like in the footer? (Currently it's a contact email and a sign up form for subscribing to updates)
• Some of the guides have a lead in paragraph before the first section starts. Is it possible to work those into the first section instead? Otherwise the design gets a bit wonky (example).

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👓 Sneak Preview of work in progress:

[zeke]

👏 Love the progress so far.

[nayafia]

Hooray, design!

Will we have a working search?

I'll let @bkeepers answer this one. My guess is no, but he hasn't yet closed out #27

What content would you like in the footer? (Currently it's a contact email and a sign up form for subscribing to updates)

Let's do "subscribe to updates" (as it is), and

# Contribute
Want to make a suggestion? This content is open source. [Help us improve it](https://github.com/github/open-source-guide/).

as an inline link, or

# Contribute
Want to make a suggestion? This content is open source. Help us improve it.
[Contribute](https://github.com/github/open-source-guide/)

^ if a button style looks better

No contact email needed.

Some of the guides have a lead in paragraph before the first section starts. Is it possible to work those into the first section instead? Otherwise the design gets a bit wonky (example).

Yep! I'll do that in a separate PR.