Create Python tools in favour of shell scripts

Python is likely to be installed when working on Python code,
which should make it more portable and consistent.

tools/docgen.py

@@ -0,0 +1,17 @@
+"""
+Run `sphinx-build` to create HTML documentation and detect errors.
+"""
+import subprocess
+import sys
+
+
+def run(*args: str) -> int:
+    return subprocess.run((sys.executable, "-m", *args)).returncode
+
+
+def main() -> None:
+    exit(run("sphinx", "-nW", "client/doc", "dist-doc"))
+
+
+if __name__ == "__main__":
+    main()