Rollup merge of rust-lang#138946 - Urgau:platform-support-bottom, r=jieyouxu

Un-bury chapters from the chapter list in rustc book

This PR moves the "Platform Support" section to the bottom of rustc chapter book, as to un-burry chapters from the chapter list, which where hidden by the wall of targets.

| Before | After |
|--------|-------|
| ![image](https://github.com/user-attachments/assets/92769307-eadb-4d9d-bdbb-9e610207eb79) | ![image](https://github.com/user-attachments/assets/1834f5c5-a1e6-4674-9be2-1094d1376eda) |

r? ``@jieyouxu``

Author: jhpratt

src/doc/rustc/src/SUMMARY.md

@@ -14,6 +14,22 @@
         - [Deny-by-default Lints](lints/listing/deny-by-default.md)
 - [JSON Output](json.md)
 - [Tests](tests/index.md)
+- [Targets](targets/index.md)
+    - [Built-in Targets](targets/built-in.md)
+    - [Custom Targets](targets/custom.md)
+    - [Known Issues](targets/known-issues.md)
+- [Profile-guided Optimization](profile-guided-optimization.md)
+- [Instrumentation-based Code Coverage](instrument-coverage.md)
+- [Linker-plugin-based LTO](linker-plugin-lto.md)
+- [Checking Conditional Configurations](check-cfg.md)
+    - [Cargo Specifics](check-cfg/cargo-specifics.md)
+- [Exploit Mitigations](exploit-mitigations.md)
+- [Symbol Mangling](symbol-mangling/index.md)
+    - [v0 Symbol Format](symbol-mangling/v0.md)
+- [Contributing to `rustc`](contributing.md)
+
+--------
+
 - [Platform Support](platform-support.md)
     - [Target Tier Policy](target-tier-policy.md)
     - [Template for Target-specific Documentation](platform-support/TEMPLATE.md)
@@ -115,16 +131,3 @@
     - [x86_64-unknown-none](platform-support/x86_64-unknown-none.md)
     - [xtensa-\*-none-elf](platform-support/xtensa.md)
     - [\*-nuttx-\*](platform-support/nuttx.md)
-- [Targets](targets/index.md)
-    - [Built-in Targets](targets/built-in.md)
-    - [Custom Targets](targets/custom.md)
-    - [Known Issues](targets/known-issues.md)
-- [Profile-guided Optimization](profile-guided-optimization.md)
-- [Instrumentation-based Code Coverage](instrument-coverage.md)
-- [Linker-plugin-based LTO](linker-plugin-lto.md)
-- [Checking Conditional Configurations](check-cfg.md)
-    - [Cargo Specifics](check-cfg/cargo-specifics.md)
-- [Exploit Mitigations](exploit-mitigations.md)
-- [Symbol Mangling](symbol-mangling/index.md)
-    - [v0 Symbol Format](symbol-mangling/v0.md)
-- [Contributing to `rustc`](contributing.md)