Update the homepage template/design

[ndiego]

Following the Make post on the Documentation redesign, I am breaking out a number of the tasks into individual issues.

One of the biggest updates will be the homepage, where the design will be updated to match the aesthetic of the newly redesigned Developer Resources homepage.

Current	Redesign (Figma)

[ryelle]

There's some missing content which should be updated before publishing this new homepage.

The Security card needs a little more text to make it onto two lines:

And we need a blurb for the Get Involved link in the footer.

[ndiego]

For Security, how about:

Everything you need to know to protect your site, from password best practices to privacy.

For Get Involved, how about:

Learn about the Documentation Team and how to contribute.

I added these to the Figma design.

[ryelle]

I've gone ahead and merged these updates (this & #87) directly, since the site also needs some page content updated and this way I can do it all at once. I'll ping for review once those page updates are complete.