docs(contrib): move docs building process to contributor guide

Author: weihanglo

src/doc/README.md renamed to src/doc/contrib/src/documentation/index.md

@@ -69,3 +69,4 @@ We'd love your help with improving the documentation! Please feel free to
 send in PRs for things you'd like to fix or change. If your change is large,
 please open an issue first, so we can make sure that it's something we'd
 accept before you go through the work of getting a PR together.
+