DOC: Link CONTRIBUTING.md to contributing.rst

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.

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Author: gfyoung