link to docs.github.com GitHub documentation site

The previous GitHub documentation site at https://developer.github.com
now has a red banner that warns "The content on this site may be out of
date" and points to the new documentation site at https://docs.github.com.
Update links in our documentation accordingly.

Reference: https://github.blog/2020-07-01-launching-docs-github-com/.

GitHub-Pull-Request: https://github.com/shurcooL/githubv4/pull/76