Disable collapsible navigation in the documentation (#5674)

Removes the collapsible sections in favor of larger font navigation
headings. I found the bold to be too distracting from the content itself
— it might be okay in the future if the navigation bar is further left.

Before:

<img width="1356" alt="Screenshot 2024-08-16 at 6 03 57 PM"
src="https://github.com/user-attachments/assets/75e49216-dc0d-4d26-a0d8-0283c29f9b81">

After:

<img width="1324" alt="Screenshot 2024-08-16 at 6 05 36 PM"
src="https://github.com/user-attachments/assets/cbce96ce-0969-46c5-80b6-e163481b8bfa">

(No change to the mobile view)

<img width="823" alt="Screenshot 2024-08-16 at 6 05 03 PM"
src="https://github.com/user-attachments/assets/b450e413-d5a4-4d2d-9905-e8eb6ac6f546">
<img width="823" alt="Screenshot 2024-08-16 at 6 05 13 PM"
src="https://github.com/user-attachments/assets/bd251ea0-58d8-456e-bdc8-4e3699061e6c">

docs/getting-started/index.md

@@ -0,0 +1,14 @@
+# Getting started
+
+To help you get started with uv, we'll cover a few important topics:
+
+- [Installing uv](./installation.md)
+- [First steps after installation](./first-steps.md)
+- [An overview of uv's features](./features.md)
+- [How to get help](./help.md)
+
+Read on, or jump ahead to another section:
+
+- Get going quickly with [guides](../guides/index.md) for common workflows.
+- Learn more about the core [concepts](../concepts/index.md) in uv.
+- Use the [reference](../reference/index.md) documentation to find details about something specific.