Release 2.0.0 - FreeRTOS SMP and 133MHz default core speed

Version 2.0.0 adds FreeRTOS SMP support thanks to @hfellner.

Normal applications will still use the default, non-RTOS
setup/loop and setup1/loop1, but for users who need preemptive
multitasking and multiple threads, FreeRTOS SMP can be used with
a simple #include <FreeRTOS.h>

Changes

• Make default CPU speed 133MHz (#557)
• FreeRTOS SMP support (#556, #555, #553, #533)
• Change the RTD theme (#554)
• Update SimpleTone.ino (#552)
• Update docs, increase max analogWriteFreq to 1MHz (#551)
• Add SoftwareSerial wrapper around SerialPIO (#548)
• Add ::overflow() return to SerialUART/SerialPIO (#547)
• Document the Generic RP2040 option (#546)