docs: document v0.1.0 installation

[jiangyuer018]

Closes #81

Problem

README.md, docs/setup.md, and SECURITY.md still said that nodkey had no release, even though v0.1.0 is published.

Change

• Put the v0.1.0 tarball and SHA256SUMS workflow first in the README quick start, with the documented verification steps.
• Keep source builds as the second installation path.
• Update setup and supported-version wording to reference the current release.

Validation

• rg -n -i 'no release' --glob '*.md' . (no matches)
• git diff --check

[groupsky]

Thank you for the PR! We fixed #81 in #85, which also changes how the systemd unit is fetched (setup guide sections 8 and 13). So I am closing this one. Thanks again for your time.