ci: sign Docker images and attach build provenance (backport of #4451)

[stevenvegt]

Backport of #4451 to V5.4. Adds Sigstore keyless signing plus SLSA provenance and SPDX SBOM attestations to the image build, so the next V5.4 patch release ships signed with a bill of materials. Users verify with:

cosign verify nutsfoundation/nuts-node:<tag> \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity-regexp '^https://github.com/nuts-foundation/nuts-node(-private)?/\.github/workflows/build-images\.yaml@'

Differences from the master change, all due to the branch's workflow layout:

• The docker job on this branch has no permissions block and uses version tags for its actions; that is left as is. The new sign job carries its own permissions: id-token: write (which drops every other permission for that job) and pins its two actions by SHA, since it is the job that holds the OIDC credential.
• V5.4 has no deployment/docker.rst; the "Verifying image signatures" section is added to the getting-started "Running on Docker" page instead.
  Verification: the workflow YAML parses, and a local Sphinx build of the docs succeeds with no warnings for the changed page. The sign job is skipped on pull_request, so the first real check is the workflow run on this branch after merge; the next tag build should be verified with the command above.

Not for release yet; part of the batch of fixes before the next V5.4 patch tag, together with #4506 and the pending backport of #4496 (version stamping).