📝 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>

.github/SECURITY.md

+# Security Policy
+
+## Supported Versions
+
+The Python team applies security fixes according to the table in
+in [the devguide](
+https://devguide.python.org/#status-of-python-branches
+).
+
+## Reporting a Vulnerability
+
+Please read the guidelines on reporting security issues [on the
+official website](
+https://www.python.org/news/security/#reporting-security-issues-in-python
+) for instructions on how to report a security-related problem to
+the Python team responsibly.
+
+To reach the response team, email `security at python dot org`.