Docs: Add CONTRIBUTING.md to docs build

Adds CONTRIBUTING.md to the docs build, and moves some
contribution-related content from getting_started.md to CONTRIBUTING.md.

Bug: 161

Change-Id: Ie5b15252eb544cbc3c42e5a0cea78ce8fe2f9f63
diff --git a/docs/BUILD.gn b/docs/BUILD.gn
index 4fa235d..3b5162b 100644
--- a/docs/BUILD.gn
+++ b/docs/BUILD.gn
@@ -41,6 +41,7 @@
   conf = "conf.py"
   sources = [
     "../CODE_OF_CONDUCT.md",
+    "../CONTRIBUTING.md",
     "../README.md",
     "index.rst",
     "modules.rst",