feat: provide GitHub attestations during release

[swetasamasti2310]

What

• Add catalog.github.active: ALWAYS to jreleaser.yml to enable GitHub Sigstore attestations for all release artifacts per the JReleaser docs
• Fix signing.armored deprecation → signing.pgp.armored (removed in JReleaser 2.0)
• Fix announce.discord deprecation → announce.webhooks.discord (removed in JReleaser 2.0)

Why

Closes #194

GitHub Attestations provide Sigstore-backed provenance for every release artifact (JARs, SBOMs, checksums), allowing users to verify that a downloaded artifact was built by the official CI workflow:

gh attestation verify docling-core-<version>.jar --repo docling-project/docling-java

##Notes
The workflow already has attestations: write and id-token: write permissions
Validated locally with jreleaser config — no config errors, no deprecation warnings
Full end-to-end verification only possible after the next real release.

[github-actions]

:java_duke: JaCoCo coverage report

Overall Project	47.54%	🔴

There is no coverage information present for the Files changed

[github-actions]

	Tests	Passed ✅	Skipped	Failed
Gradle Test Results (all modules & JDKs)	1792 ran	1792 passed	0 skipped	0 failed

Test	Result
No test annotations available

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[github-actions]

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