Add a script that builds a static Linux executable

scripts/build-static.sh builds the daemon for x86_64 Linux against
musl, as one executable without dependencies to copy to a router. It
finds a musl C compiler for jemalloc (musl-cross on macOS, musl-gcc on
Linux), and uses its ar and ranlib too: with the host's, a build from
macOS gets an empty libjemalloc.a and fails to link. It prints the
revision and the SHA-256 of the result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/content/docs/getting-started/installation.md

@@ -57,6 +57,35 @@ To build and install the daemon:
 
    The command displays the installed version, for example `bgp 0.1.0`.
 
+## Building a Static Executable
+
+The script `scripts/build-static.sh` builds the daemon as a single static
+executable for x86_64 Linux, linked against musl. The executable runs on any
+x86_64 Linux distribution without other dependencies.
+
+The script requires the Rust target and a musl C compiler for jemalloc:
+
+```bash
+rustup target add x86_64-unknown-linux-musl
+```
+
+On macOS, install the cross compiler with
+`brew install FiloSottile/musl-cross/musl-cross`. On x86_64 Linux, install
+`musl-tools` (Debian, Ubuntu) or `musl` (Arch Linux).
+
+```bash
+scripts/build-static.sh
+```
+
+The script displays the path of the executable, the source revision, and
+the SHA-256 checksum of the executable:
+
+```
+target/x86_64-unknown-linux-musl/release/bgp: ELF 64-bit LSB pie executable, x86-64, static-pie linked
+revision 9ce81a8
+2faa32d3...  target/x86_64-unknown-linux-musl/release/bgp
+```
+
 ## Building a Container Image
 
 The `Dockerfile` in the repository builds the daemon in a Rust build stage