docs: refer to STM32F429I-DISC1 in express setup
docs: refer to STM32F429I-DISC1 in express setup

- Adds a reference earlier on in the Getting Started guide to the
  supported STM32F429I-DISC1 Discovery Kit during the Express
  Setup to mention what hardware will be used later in case the
  user wants to follow along in person.

Change-Id: I82df43f00da0acd06939c306004cc8ffd121b61f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/13820
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

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