docs: Add dark mode (#17940)

Closes https://github.com/zed-industries/zed/issues/17911

This PR enables dark mode on the documentation. This is done without any
special plugins, just pure JavaScript and CSS variables. I may open
fast-follow PRs to fine-tune design and code details that haven't been
super polished yet. For example, when switching to dark mode, the
`class` attribute on the `html` tag would change immediately, whereas
other attributes such as `data-theme` and `data-color-scheme` would
require a full refresh. This seems to be resolved, but not 100%
confident yet.

---

Release Notes:

- Enabled dark mode on the documentation

docs/theme/plugins.css

@@ -1,5 +1,5 @@
 kbd.keybinding {
-  background-color: #f0f0f0;
+  background-color: var(--keybinding-bg);
   padding: 2px 4px;
   border-radius: 3px;
   font-family: monospace;