Add pin pull-down example

[mutantbob]

Based on your comments and the code you linked to, I created an example that seems to work on my Teensy 4.0

[mciantyre]

Thank you! Example looks great.

[mciantyre]

bors try

[bors[bot]]

try

Build succeeded:

• precompiled
• format
• test
• doclinks
• clippy
• xplat (ubuntu-latest)
• xplat (macos-latest)
• template
• xplat (windows-latest)
• doc