Relicense PliegoRS under GPL v3 and CC BY-SA 4.0

[terrizoaguimor]

Summary

• relicense future PliegoRS software revisions under GPL-3.0-only
• license project-authored documentation and original non-software media under CC-BY-SA-4.0
• preserve historical Apache-2.0 grants for v0.4.0-beta.1 and earlier revisions
• keep generated application ownership explicit with a neutral LICENSE.md placeholder while retaining GPL coverage for copied template code
• update crates metadata, SPDX headers, public site copy, release bundles, and contribution policy
• add an executable repository license-contract gate to CI

Verification

• cargo fmt --all -- --check
• cargo test -p pliego-starters --locked
• cargo test -p pliego-cli --test cli_contract --locked
• cargo clippy -p pliego-starters -p pliego-cli --all-targets -- -D warnings
• npm run check:product-truth
• npm run check:licenses
• npm run check:distribution
• npm run check:docs
• npm run check:site
• node --test scripts/check-distribution.test.mjs
• cargo package --allow-dirty --no-verify -p pliego-dom
• cargo package --allow-dirty --no-verify -p pliego-starters
• git diff --check