Add search boards via primary processor.

[water5]

From circuitpython.org/downloads page, add primary processor used on board option to 'Filters' likes 'SAMD21, SAMD51, ESP32-S2, STM32, nRF52840, RP2040 ...'.
Currently, can't to do that, e.g. : input 'RP2040' to 'Search for CircuitPython boards' field, the 'Pico' can't list out,
input 'ESP32-S2' to 'Search for CircuitPython boards' field, 'Saola 1 w/WROVER, Saola 1 w/WROOM' can't list out.

[makermelissa]

Huh, this is an interesting idea. I wonder if we can add something to the data files that CircuitPython generates since every board is already split up in Ports folders.

[makermelissa]

Although SAMD21 and SAMD51 are grouped together in CP, so maybe not the best approach. If we add to the data files, I think we'll have to have some checks in place so the values are restricted to a list because Jekyll likes to group things like "SAMD21", "samd21", and "samd 21" differently.

[tannewt]

In the long term I'd like to see a TOML database for boards similar to https://github.com/adafruit/nvm.toml. That info is more broadly useful than just having it in CP.