docs: Move workshop content to //examples

Reformat README.md files to rst and create a 'docs' build group.

Change-Id: Icf1df5550deb203415f6996b4552204074210dee
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/168343
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Anthony DiGirolamo <tonymd@google.com>
diff --git a/.gn b/.gn
index e6a1f60..0868eed 100644
--- a/.gn
+++ b/.gn
@@ -23,4 +23,6 @@
 
   # Default gn build virtualenv target.
   pw_build_PYTHON_BUILD_VENV = "//:sample_project_build_venv"
+
+  pw_build_PIP_REQUIREMENTS = [ "//docs/requirements.txt" ]
 }