Count HACS downloads with release ZIPs and a README badge

[powerassistant]

HACS currently downloads the integration from repository sources, so its downloads have no GitHub release-asset counter. This change routes new releases through a named ZIP and displays its cumulative download count in the README.

Changes:

• Enable zip_release with siemens_sentron.zip and prepare version V26.09.10 consistently.
• Add a deterministic package builder with the flat directory structure HACS requires.
• Add Prepare release, a manually started workflow that runs the existing four validation jobs, checks the version, and creates a new tag on the validated commit plus a draft release with its ZIP attached.
• Add a README badge that counts this ZIP across releases, and update the installation and release instructions.

Validation:

• Repository validator passed locally.
• All 68 tests passed locally, including nine ZIP packaging tests covering extraction layout, version mismatch, missing files, cache exclusion, symlinks, deterministic output and overwrite prevention.
• Workflow YAML and shell syntax checked locally.
• All four GitHub checks passed on the final commit c27b99e: Repository checks, HACS, Hassfest and Home Assistant runtime import. Validation run.

To activate after merging: open Actions → Prepare release → Run workflow, choose main, enter V26.09.10 and release notes. Review the generated draft under Releases, then publish it with siemens_sentron.zip already attached.

Counts start with downloads of the new asset and include updates and repeated downloads. Earlier source downloads cannot be recovered from these counters. Existing releases retain their original source-download configuration. The integration's device communication and controls are unchanged.

Icon investigation: both existing PNGs are valid and included in the ZIP. The HACS 2.0.5 overview still uses the older central brands CDN, which has no entry for siemens_sentron. There is no supported integration-side configuration to override that list icon. The upstream change is tracked in hacs/integration#5388; the README now explains this limitation. This PR does not claim to fix that upstream HACS display issue.