docs(template): release notes template

[hkimw]

Summary

• add an orphan MyST release notes template for future pccx versions
• include release metadata, validation, limitations, milestones, and release checklist placeholders

Tests

• make strict SPHINXBUILD=/tmp/pccx-coc-wt/.venv/bin/sphinx-build
• PATH=/tmp/pccx-coc-wt/.venv/bin:/home/hwkim/.local/bin:/home/hwkim/bin:/home/hwkim/.codex/tmp/arg0/codex-arg0G9WI10:/home/hwkim/.nvm/versions/node/v24.14.0/lib/node_modules/@openai/codex/node_modules/@openai/codex-linux-x64/vendor/x86_64-unknown-linux-musl/path:/home/hwkim/.local/bin:/home/hwkim/.local/bin:/home/hwkim/.local/bin:/home/hwkim/.nvm/versions/node/v24.14.0/bin:/tools/Xilinx/2025.2/Model_Composer/bin:/tools/Xilinx/2025.2/Vivado/bin:/tools/Xilinx/2025.2/PDM/bin:/tools/Xilinx/2025.2/Vitis/bin:/tools/Xilinx/2025.2/Vitis/bin:/tools/Xilinx/2025.2/Vitis/gnu/riscv/linux_toolchain/lin32/bin:/tools/Xilinx/2025.2/Vitis/gnu/riscv/linux_toolchain/lin64/bin:/tools/Xilinx/DocNav:/home/hwkim/.cargo/bin:/home/hwkim/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/snap/bin:/home/hwkim/.claude/plugins/cache/claude-plugins-official/rust-analyzer-lsp/1.0.0/bin:/home/hwkim/.claude/plugins/cache/claude-plugins-official/figma/2.1.30/bin make lint

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