Move Rust and Python version support out of the Platform support policy (#18535)

The content is unchanged.

I'm planning to expand the content in the Python version and Platform
support files and generally don't like that these are mixed.

docs/reference/policies/python.md

@@ -0,0 +1,37 @@
+# Python support
+
+## Python versions
+
+uv supports and is tested against the following Python versions:
+
+- 3.8
+- 3.9
+- 3.10
+- 3.11
+- 3.12
+- 3.13
+- 3.14
+
+## Python implementations
+
+uv has Tier 1 support for the following Python implementations:
+
+- CPython
+
+As with [platforms](./platforms.md), Tier 1 support can be thought of "guaranteed to work". uv
+supports managed installations of these implementations, and the builds are maintained by Astral.
+
+uv has Tier 2 support for:
+
+- PyPy
+- GraalPy
+
+uv is "expected to work" with these implementations. uv also supports managed installations of these
+Python implementations, but the builds are not maintained by Astral.
+
+uv has Tier 3 support for:
+
+- Pyston
+- Pyodide
+
+uv "should work" with these implementations, but stability may vary.