📝 Add a GitHub-specific security page (GH-13526)

* 📝 Add a GitHub-specific security page

It will show up @
https://github.com/python/cpython/security/policy
allowing to navigate users who get there from "Security" tab in the
GitHub repo to the full article explaining the security vulnerability
reporting practices.

Co-Authored-By: Hugo <hugovk@users.noreply.github.com>
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