Port more content to website-next (#1435)

Summary:
This builds upon https://github.com/facebook/yoga/pull/1433 and starts porting over and fixing some of the code in website-next.

1. Create a hero similar to current https://yogalayout.com hero
2. Start moving from `antd` and harcoded colors to [Infima](https://infima.dev/docs/getting-started/introduction/) primitives provided by Docusaurus
3. Replaced some more stock docusaurus assets, links, and text with the ones for Yoga.

There is still a lot to do here (not the least, adding real content), but it's beginning to look like a website, and is already pretty snappy. Eventually I want to get SSR working correctly with Playground, which is still a little broken in the port.

Pull Request resolved: https://github.com/facebook/yoga/pull/1435

Test Plan:
**Gatsby Original**
<img width="1795" alt="image" src="https://github.com/facebook/yoga/assets/835219/7670d53a-00a8-4146-a100-e4a05dd77488">

**New (light mode)**
<img width="800" alt="image" src="https://github.com/facebook/yoga/assets/835219/ebe11d15-5f6f-445f-bcc8-9ec51ecfac62">

**New (dark mode)**
<img width="800" alt="image" src="https://github.com/facebook/yoga/assets/835219/ca44a492-46df-410a-8303-baec3029ec49">

Reviewed By: yungsters

Differential Revision: D50523462

Pulled By: NickGerleman

fbshipit-source-id: 61b4610104f695a4e38a7d4bb6a0c2488bd6f89e

website-next/src/components/Playground/Sidebar.module.css

@@ -0,0 +1,21 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+ .placeholder {
+  color: var(--ifm-color-content-secondary);
+  margin: auto;
+  text-align: center;
+ }
+
+ .sidebar {
+  z-index: 3;
+  width: 320px;
+  background: var(--ifm-background-surface-color);
+  display: flex;
+  flex-direction: column;
+  padding: 20px;
+}