feat(docs): add dark theme switcher

[evwilkin]

Closes #2885
Closes #2959

This PR adds a dark theme switcher to the website. No designs to work from so open to feedback.

Note: current code will expose switcher on the live site & not in the workspaces. Once design is confirmed & approved, we'll need to edit the patternfly-docs.config.js file in each repo separately to add the hasDarkThemeSwitcher property to the exports & set it to true (to show the switcher) or false (to not render the switcher).

[patternfly-build]

PF4 preview: https://patternfly-org-pr-3014-v4.surge.sh/v4

[evwilkin]

FYI @mcoker @nicolethoen I've updated this PR to disable the theme switcher in org (hasDarkThemeSwitcher = false) but this adds the mechanism to pull it in & enable is separately in core & react 🪄

[septatrix]

FYI @mcoker @nicolethoen I've updated this PR to disable the theme switcher in org (hasDarkThemeSwitcher = false) but this adds the mechanism to pull it in & enable is separately in core & react magic_wand

Why did you disable this? Is it because it is not yet fully supported?

[evwilkin]

@septatrix dark theme was added in a Beta status as it was built out specifically around the needs of particular product, and does not yet have widespread support across all PF components. Additionally, our website doesn't have any built-in support for displaying dark theme correctly, so while some components would look correct there are many other things that would need to be updated (screenshots, limiting which areas of the page are affected by a theme toggle, etc) that need to be considered & updated for the site to effectively display dark theme examples.

We are currently planning on how to revamp theming within PatternFly & roll out official support for an updating system with our next major release, and will be discussing website updates in the coming weeks, so hope to have a more concrete plan around all of this later this year.