doc: make doc testable

doc/_readme/scripts/install_llgo.sh

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+# shellcheck disable=all
+git clone https://github.com/goplus/llgo.git
+cd llgo
+go install -v ./cmd/...
+go install -v ./chore/...  # compile all tools except pydump
+cd chore/_xtool
+llgo install ./...   # compile pydump
+go install github.com/goplus/hdq/chore/pysigfetch@v0.8.1  # compile pysigfetch

doc/_readme/scripts/install_macos.sh

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+# shellcheck disable=all
+brew update
+brew install llvm@18 pkg-config bdw-gc openssl cjson
+brew install python@3.12 # optional
+go install -v github.com/goplus/llgo/cmd/llgo@latest

doc/_readme/scripts/install_ubuntu.sh

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+# shellcheck disable=all
+echo "deb http://apt.llvm.org/$(lsb_release -cs)/ llvm-toolchain-$(lsb_release -cs)-18 main" | sudo tee /etc/apt/sources.list.d/llvm.list
+wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+sudo apt-get update
+sudo apt-get install -y llvm-18-dev clang-18 libclang-18-dev lld-18 pkg-config libgc-dev libssl-dev zlib1g-dev libcjson-dev python3.12-dev
+sudo apt-get install -y python3.12-dev # optional
+go install -v github.com/goplus/llgo/cmd/llgo@latest

doc/_readme/scripts/run.sh

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+#!/bin/bash
+cd ./doc/_readme/ || exit 1
+llgo build -v ./...
+pip3 install --user numpy
+for dir in ./*/; do
+  if grep -q "func main()" "$dir"/*.go 2>/dev/null; then
+    echo "Running examples in $dir"
+    llgo run "$dir"
+  fi
+done