docs: Add a new variant (board) and "Generic System Clock configuration"

[thernstig]

I had a hard time understanding https://github.com/stm32duino/Arduino_Core_STM32/wiki/Add-a-new-variant-%28board%29#3---generic-system-clock-configuration.

I am quoting the parts I found hard and what I felt was missing/strange:

Go to Pinout tab, enable the desired peripherals which require specific clock configuration (not needed for peripherals clocked by HCLKx, or APBx clock): SDIO, USB, ... In this example only USB needs to be enabled as other peripherals default clock are correct by default.

What is "desired peripherals" here? How would I know what to enable?

Set PLL Source Mux to HSI.

There is no PLL Source Mux for the MCU I added in #2556. The sentence is written like it is something that should exist, but it does not seem true for all MCUs.