Arduino_core_stm32: Add documentation for the particularities of this core

[romainreignier]

To follow the discussion started in #458, it would be nice to document the usage of this core for users coming for official Arduino core.

Even if a dedicated website would be nice, it requires some work to setup a template with some style, categories... So instead, I propose to leverage the existing wiki.

I have started by adding the HardwareSerial info reported in #458, see here:
https://github.com/stm32duino/wiki/wiki/API#hardwareserial

Do you think it is the right place? I am open to any suggestions.

@fpistm For the HardwareSerial case, using the first method (declaring a HardwareSerial in the sketch) will not enable the serialEvent() callback, right?
For what I understand here:
https://github.com/stm32duino/Arduino_Core_STM32/blob/830bb00c7ee54fe8b7a0f7ca6edd5a8b44e5fbc7/cores/arduino/WSerial.h#L33-L37

Only passing -DENABLE_HWSERIAL1 will enable it. Am I right?

ToDo:

• [ ] [Core debug](http://stm32duino.com/viewtopic.php?f=48&t=4700&hilit=core+debug)
• [ ] Pin mapping vs Pin number
• [ ] Digital(Read|Write)Fast(), digitalToggle() and digitalToggleFast()
  ...

[fpistm]

Hi @romainreignier and thanks.
you're right for serialEvent.
Serial is the generic one map to the default serial instance defined in the variant if any.

[romainreignier]

@fpistm Thank you, I have added a note.

So if you agree with the format, I will had the same kind of section for SPI and i2c.

[fpistm]

Agreed of course 👍

[romainreignier]

I have added i2c and SPI examples.

[fpistm]

~Note for me: update upload method with #514~ --> done
https://github.com/stm32duino/wiki/wiki/Upload-methods#stm32cubeprogrammer

[fpistm]

Since a month I've added/updated several Wiki pages.
I've added a ToDo list in the OP to track what is missing.

[fpistm]

Wiki is updated regularly as requested by the OP.
I close this one as it is not used to track the Wiki which is updated in a dedicated repo.