doc: Add release notes describing bitcoin wrapper executable

doc/release-notes-31375.md

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+New command line interface
+--------------------------
+
+A new `bitcoin` command line tool has been added to make features more
+discoverable and convenient to use. The `bitcoin` tool just calls other
+executables and does not implement any functionality on its own.  Specifically
+`bitcoin node` is a synonym for `bitcoind`, `bitcoin gui` is a synonym for
+`bitcoin-qt`, and `bitcoin rpc` is a synonym for `bitcoin-cli -named`. Other
+commands and options can be listed with `bitcoin help`. The new tool does not
+replace other tools, so all existing commands should continue working and there
+are no plans to deprecate them.