doc: Move doxygen intro to file for USE_MDFILE_AS_MANPAGE

doc: Change header to notitle

doc/README_doxygen.md

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+\mainpage notitle
+
+\section intro_sec Introduction
+
+This is the developer documentation of the reference client for an experimental new digital currency called Bitcoin,
+which enables instant payments to anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate
+with no central authority: managing transactions and issuing money are carried out collectively by the network.
+
+The software is a community-driven open source project, released under the MIT license.
+
+See https://github.com/bitcoin/bitcoin and https://bitcoincore.org/ for further information about the project.
+
+\section Navigation
+Use the buttons <code>Namespaces</code>, <code>Classes</code> or <code>Files</code> at the top of the page to start navigating the code.
+