docs: clarify Quick Start toolchain requirements

[124107157-KV]

Summary

Clarify the software toolchain requirements in the CORE-V-MCU Quick Start
Guide.

The Quick Start Guide uses the pre-built cli_test.srec supplied with the
repository, so users following the pre-built flow do not need to install a
compiler toolchain.

Users who want to build or modify cli_test from source are now directed to
the core-v-mcu-cli-test repository for the CORE-V GCC toolchain and software
setup instructions.

Validation

• git diff --check
• Verified that only emulation/quickstart/README.md is modified
• Verified the core-v-mcu-cli-test documentation link

Fixes #226.

[MikeOpenHWGroup]

Thanks for the contribution @124107157-KV - looks good. The lint check has yet to complete, but I will merge anyway because the linter checks SystemVerilog sources and this is a documentation-only update.