Update setup section for std on riscv32imac MCUs (esp-rs#100)

* Update setup section for std on riscv32imac MCUs

Massage setup section to include directions for riscv32imac-esp-espidf targets

* Add link to rustc book for esp-idf nostd targets

* Change to fancier link syntax

Change to link syntax as used elsewhere on this page, reference the nightly rustc book which has the necessary page.

src/installation/index.md

@@ -59,14 +59,17 @@ To build Rust applications for the Espressif chips based on `RISC-V` architectur
 
       This target is currently [Tier 2][rust-lang-book--platform-support-tier2]; note the different flavors of `riscv32` target in Rust covering different [`RISC-V` extensions][wiki-riscv-standard-extensions].
 
-    - For `std` applications, use the target `riscv32imc-esp-espidf`.
-
-      Since this target is currently [Tier 3][rust-lang-book--platform-support-tier3], it does not have pre-built objects distributed through `rustup`, and **it does not need to be installed** as the `no_std` target.
+    - For `std` applications:
 
+      Since this target is currently [Tier 3][rust-lang-book--platform-support-tier3], it does not have pre-built objects distributed through `rustup` and, unlike the `no_std` target, **nothing needs to be installed**. Refer to the [*-esp-idf][rust-lang-book--platform-support--esp-idf] section of the rustc book for the correct target for your device. 
+
+      - `riscv32imc-esp-espidf` for MCUs which do not support atomics, like ESP32-C2 and ESP32-C3
+      - `riscv32imac-esp-espidf` for MCUs which support atomics, like ESP32-C6, ESP32-H2, and ESP32-P4
 
 [rust-lang-book--platform-support-tier2]: https://doc.rust-lang.org/nightly/rustc/platform-support.html#tier-2
 [wiki-riscv-standard-extensions]: https://en.wikichip.org/wiki/risc-v/standard_extensions
 [rust-lang-book--platform-support-tier3]: https://doc.rust-lang.org/nightly/rustc/platform-support.html#tier-3
+[rust-lang-book--platform-support--esp-idf]: https://doc.rust-lang.org/nightly/rustc/platform-support/esp-idf.html
 
 
 3. To build `std` projects, you also need to install: