Bump the version to 5.7.0 and write the release section of the changelog

[avgvi]

The version bump for the first release under the diskcache-community name.

• diskcache/__init__.py: __version__ 5.7.0, __build__ 0x050700. pyproject.toml and docs/conf.py read the version from there.
• CHANGELOG.md: Unreleased becomes 5.7.0 (unreleased) with an introduction (what the release is, what stays compatible) and entries for PRs 12, 13 and 14; the release workflow publishes this section as the GitHub release notes.
• README.rst: the fork notice says 5.7.0 is the first release instead of "no release has been published yet".
• SECURITY.md: the CVE section describes 5.7.0 (default Disk unchanged, opt-in SafeDisk, 0700 directories) instead of the state of master, and the supported-versions text drops the pre-release sentence.

Do not merge until the pypi and testpypi environments exist with their required reviewer, the pending trusted publishers are registered on PyPI and TestPyPI, the publish action is on the Actions allow-list, and the TestPyPI dry run has passed. Right before merging, the heading gets its date. The v5.7.0 tag on the merge commit then triggers the release workflow.

Verified locally: python -m build produces diskcache_community-5.7.0 sdist and wheel, twine check --strict passes, rstcheck, doc8 and the Sphinx build pass, 357 tests pass, and import diskcache reports 5.7.0.