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/css/general.css

@@ -174,10 +174,10 @@ h6:target::before {
 }
 .content a {
   text-decoration: underline;
-  text-decoration-color: hsl(219, 93%, 42%, 0.2);
+  text-decoration-color: var(--link-line-decoration);
 }
 .content a:hover {
-  text-decoration-color: hsl(219, 93%, 42%, 0.5);
+  text-decoration-color: var(--link-line-decoration-hover);
 }
 .content img,
 .content video {
@@ -219,7 +219,7 @@ table thead td {
 }
 table thead th {
   padding: 6px 12px;
-  color: #000;
+  color: var(--full-contrast);
   text-align: left;
   border: 1px var(--table-border-color) solid;
 }
@@ -235,7 +235,7 @@ blockquote {
   margin: auto;
   margin-top: 1rem;
   padding: 1rem 1.25rem;
-  color: #000;
+  color: var(--full-contrast);
   background-color: var(--quote-bg);
   border: 1px solid var(--quote-border);
 }
@@ -315,7 +315,7 @@ kbd {
   font-size: 1.4rem;
   margin: 0.5em 0;
   border-bottom: 1px solid;
-  border-color: var(--border-light);
+  border-color: var(--divider);
 }
 .footnote-definition p {
   display: inline;
@@ -356,6 +356,6 @@ kbd {
 }
 
 code.hljs {
-  color: hsl(221, 13%, 10%) !important;
-  background-color: hsla(221, 93%, 42%, 0.1);
+  color: var(--code-text) !important;
+  background-color: var(--code-bg);
 }