fix: add platform support top level docs

Author: nhtyy

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

@@ -367,6 +367,7 @@ target | std | host | notes
 `riscv32gc-unknown-linux-gnu` | ✓ |   | RISC-V Linux (kernel 5.4, glibc 2.33)
 `riscv32gc-unknown-linux-musl` | ? |   | RISC-V Linux (kernel 5.4, musl 1.2.3 + RISCV32 support patches)
 [`riscv32im-risc0-zkvm-elf`](platform-support/riscv32im-risc0-zkvm-elf.md) | ? |  | RISC Zero's zero-knowledge Virtual Machine (RV32IM ISA)
+[`riscv32im-succinct-elf`](platform-support/riscv32im-succinct-elf.md) |  |  |
 [`riscv32ima-unknown-none-elf`](platform-support/riscv32-unknown-none-elf.md) | * |  | Bare RISC-V (RV32IMA ISA)
 [`riscv32imac-esp-espidf`](platform-support/esp-idf.md) | ✓ |  | RISC-V ESP-IDF
 [`riscv32imac-unknown-nuttx-elf`](platform-support/nuttx.md) | ✓ |  | RISC-V 32bit with NuttX