Migrate layout to the destination layout

First I imported the commits from v3.6.6 with:

  PKG=devices
  git init $PKG
  cd $PKG
  git remote add origin https://github.com/google/periph
  git fetch origin
  git filter-repo \
    --path AUTHORS \
    --path CONTRIBUTING.md \
    --path CONTRIBUTORS \
    --path LICENSE \
    --path README.md \
    --path $PKG --path experimental/$PKG $@
  git remote add origin git@github.com:periph/$PKG
  git fetch origin
  # Use the occasion to rename from master to main since we are starting from
  # scratch.
  git branch -m main
  git push origin main

Then I fixed the layout to get rid of experimental with:

  git rm experimental/devices/README.md
  git mv experimental/devices/* .
  git mv devices/* .

The import paths will be fixed in a follow up.

experimental/devices/README.md

@@ -1,5 +0,0 @@
-# experimental/devices
-
-You are welcome to send PR (pull request) to add experimental device drivers
-here. Please follow the instructions in
-[project/contributing/](https://periph.io/project/contributing/).