updated docs/third_party.md (#19374)

Added link to `docs/third_party.md` with an alternative C++ implementation of protobuf called [simple-protobuf](https://github.com/tonda-kriz/simple-protobuf)

Closes #19374

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/19374 from tonda-kriz:main 8fd382d9c6
PiperOrigin-RevId: 700232574

docs/third_party.md

@@ -22,6 +22,7 @@ These are projects we know about implementing Protocol Buffers for other program
 *   C: https://github.com/eerimoq/pbtools
 *   C++: https://github.com/protocolbuffers/protobuf (Google-official
     implementation)
+*   C++: https://github.com/tonda-kriz/simple-protobuf
 *   C++: https://EmbeddedProto.com
 *   C++: https://github.com/yksten/struct2x
 *   C/C++: http://spbc.sf.net/