docs: Use plain quotes for callouts (#9499)

This PR adjusts our docs to use plain Markdown quotes for callouts.

This Markdown gets rendered in a number of different locations:

- GitHub
- zed.dev
- mdBook

As such, we should use Markdown that is supported by all three.

Release Notes:

- N/A

docs/src/SUMMARY.md

@@ -1,6 +1,7 @@
 # Summary
 
 [Getting Started](./getting_started.md)
+  - [System Requirements](./system_requirements.md)
 [Feedback](./feedback.md)
 
 # Configuring Zed