S23.06: Documentation tarball as release artefact

[DavidCozens]

Parent epic: #246

Goal

Attach a self-contained docs tarball to every GitHub Release so that
customers have an offline browseable copy as supply-chain evidence.

Tasks

• Extend the release workflow so the docs build from S23.05 is also
  packaged as solid-syslog-docs-vX.Y.Z.tar.gz.
• The tarball must be self-contained HTML — extract and open
  index.html should work offline with no external resources fetched
  from CDNs. (Sphinx themes vary on this — verify by extracting in an
  airgapped sandbox or with network disabled and checking the page
  renders fully.)
• Attach to the GitHub Release using gh release upload (or
  softprops/action-gh-release — match what SBOM uses).
• Verify the tarball appears alongside the SBOM and source tarball on
  the release page.
• Update DEVLOG.md.

Done when

• Cutting a stable release attaches solid-syslog-docs-vX.Y.Z.tar.gz
  to the release.
• The tarball renders fully when extracted offline.
• It is listed on the release page next to the SBOM and source
  tarball.