DOC: Link CONTRIBUTING.md to contributing.rst (#15451)

Previously, we were trying to maintain two different copies of the documentation, one in the ".github" directory, and the other in the "docs/," which just imposes greater maintenance burden in the long-run.  We now use the ".github" to refer to different portions of the "docs/" version with short summaries for ease of navigation.

Closes gh-15349.

Author: gfyoung