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.

Fixes #194

[github-actions]

:java_duke: JaCoCo coverage report

Overall Project	48.12%	🔴

There is no coverage information present for the Files changed

[github-actions]

	Tests	Passed ✅	Skipped	Failed
Gradle Test Results (all modules & JDKs)	1844 ran	1844 passed	0 skipped	0 failed

Test	Result
No test annotations available

• View Gradle Test Results (all modules & JDKs)

[github-actions]

HTML test reports are available as workflow artifacts (zipped HTML).

• Download: Artifacts for this run

[edeandrea]

Thanks @swetasamasti2310 !

Could you please fix the commit sign-off so that the DCO check will pass? You can take a look at https://github.com/docling-project/docling-java/pull/640/checks?check_run_id=97043886512 for instructions on how to do that.

[swetasamasti2310]

Thanks @swetasamasti2310 !

Could you please fix the commit sign-off so that the DCO check will pass? You can take a look at https://github.com/docling-project/docling-java/pull/640/checks?check_run_id=97043886512 for instructions on how to do that.

@edeandrea Fixed the commit sign-off. Please check if it can be merged.

[github-actions]

HTML test reports are available as workflow artifacts (zipped HTML).

• Download: Artifacts for this run

[edeandrea]

@swetasamasti2310 does anything need to be done during the release CI workflow? Anything from https://github.com/actions/attest?

Does jreleaser do that automatically? Or is jreleaser just publishing as part of the release?