Rollup merge of rust-lang#151689 - zzaekkii:xtensa-installation, r=jieyouxu

Fix broken Xtensa installation link

### Location (URL)
https://doc.rust-lang.org/rustc/platform-support/xtensa.html

<img width="800" alt="image" src="https://github.com/user-attachments/assets/dbf1fea5-e65f-4bb2-beea-bf3267b12aff" />

### Summary
The Xtensa platform documentation currently links to an outdated Rust on ESP Book installation page that no longer exists.

The Rust on ESP Book has been reorganized, and the installation instructions previously referenced under `/book/installation/` are now located under the Getting Started section.

The link is updated to reference the current Toolchain Installation page, which contains the up-to-date instructions for building Xtensa targets.

Author: Zalathar

src/doc/rustc/src/platform-support/xtensa.md

@@ -24,4 +24,4 @@ Xtensa targets that support `std` are documented in the [ESP-IDF platform suppor
 
 ## Building the targets
 
-The targets can be built by installing the [Xtensa enabled Rust channel](https://github.com/esp-rs/rust/). See instructions in the [RISC-V and Xtensa Targets section of The Rust on ESP Book](https://docs.espressif.com/projects/rust/book/installation/index.html).
+The targets can be built by installing the [Xtensa enabled Rust channel](https://github.com/esp-rs/rust/). See instructions in the [RISC-V and Xtensa Targets section of The Rust on ESP Book](https://docs.espressif.com/projects/rust/book/getting-started/toolchain.html).