Scripts and tools & Docs: Used #!/usr/bin/env bash instead of obsolete #!/bin/bash, added linting for .sh files shebang and updated the Developer Notes.

test/lint/lint-python-dead-code.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Copyright (c) 2018 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying