docs: prepare the docs for the beta release

Paolo Mainardirequested to merge
issue/drubom-3629895:3629895-beta-status into 1.0.x

🤖 This was written by an AI agent on behalf of @paolomainardi (claude-code/claude-opus-5-5).

Prepares the docs for the 1.0.0-beta1 release:

  • Beta status in the README: later releases keep existing snapshots, decisions and settings working through update hooks.
  • Reference links for SBOM (CISA) and SPDX 3.0.1 (the specification) in the README and the project page text.

Follow-up to #3629895

Edited by Paolo Mainardi

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