Refactor theme code, add an option to forget a saved theme, and reorder content

[AlexOwen]

This PR improves the theme code, keeping the assumption that the designer wants to pair a dark page with a light theme and vice versa. It also reorganises the content to provide a better separation of functions.

User stories affected/created:

• As a user I want to change the website theme
• As a user I want the website theme to match my system theme

New features:

• Adds an option to forget theme preference

Changes:

• Fixes animations on page load, and moves them to a separate section of the CSS for easy maintenance
• Moves the theme controls to the footer and the contributors to the body, because the contributors are part of the core content of this page while the theme controls are less important
• Replaces theme switching code with a more functional approach
• Adds error checking to theme switching code where necessary
• Moves custom script loading to the head, so there is no flash on page load when a non-default theme is loaded (supersedes https://github.com/oxidecomputer/design-site/pull/44)
• Adds documentation to custom functions

[vercel[bot]]

This pull request is being automatically deployed with Vercel (learn more). To see the status of your deployment, click on the icon next to each commit.