Merge pull request #1820 from westbaystars/add-doxygen-to-rp2-prerequisites

Add doxygen to list of RP2 prerequisites

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

Author: bettio

doc/src/build-instructions.md

@@ -840,6 +840,7 @@ You can build with all boards supported by Raspberry Pi pico SDK, including Pico
 ### RP2 Prerequisites
 
 * `cmake`
+* `doxygen`
 * `ninja`
 * `Erlang/OTP`
 * `Elixir` (optional)