> Is the Raspberry Pi an open-source software and hardware platform?

No, the hardware isn't open, and not all of the software is, either. It requires a closed-source blob of code to boot the system. There's "real" firmware in the VC4 chip, which is the first part of the Pi to come up, some binary blob bootloader files, and drivers handling graphics and hardware decoding that are still closed. There is an open-source 3D driver, but it's missing some capabilities from the closed one (hardware accelerated video decoding), but includes some extras (some degree of support for non-mobile OpenGL).

The chips from most other vendors are at least bootable without blobs, even if video output won't be properly accelerated.