Organise chapters in mdbook sidebar

Author: Wilfred

Diff for: src/tools/rust-analyzer/docs/book/src/SUMMARY.md

@@ -4,10 +4,10 @@
 - [Installation](installation.md)
 - [Troubleshooting](troubleshooting.md)
 - [Configuration](configuration.md)
-- [Non-Cargo Based Projects](non_cargo_based_projects.md)
+  - [Non-Cargo Based Projects](non_cargo_based_projects.md)
 - [Security](security.md)
 - [Privacy](privacy.md)
 - [Features](features.md)
-- [Assists (Code Actions)](assists.md)
-- [Diagnostics](diagnostics.md)
+  - [Assists (Code Actions)](assists.md)
+  - [Diagnostics](diagnostics.md)
 - [Editor Features](editor_features.md)