Add contributor templates, PR labeler, and release-note categories

[frazane]

Adds the contributor-facing automation the repo was missing: issue and PR templates, automatic path-based PR labels, and categorized release notes. For a library that runs across multiple backends, the bug template in particular nudges reporters to tell us which backend and versions they hit.

What changed

• Issue forms under .github/ISSUE_TEMPLATE/: a bug report that asks for backend, Python version, scoringrules version and a minimal reproducer, plus a feature request form. Blank issues are disabled.
• .github/PULL_REQUEST_TEMPLATE.md with a short checklist (tests across backends, both array-API and numba paths, docs, release-note label).
• .github/release.yml so GitHub's auto-generated release notes are grouped by label, including a Breaking changes section at the top.
• A path-based labeler (.github/labeler.yml + a labeler workflow) that tags PRs as core / docs / ci / backend:jax / backend:torch from the files they touch. These labels feed the release-note categories.

Notes

• The labeler runs on pull_request_target so it can label PRs from forks; it only reads the changed-file list and applies labels, it doesn't run PR code.
• Release-note categorization keys off labels on the merged PRs, so cutting a release with good notes means the merged PRs need the right labels applied.

[Copilot]

Pull request overview

This PR adds contributor-facing GitHub configuration and automation to improve issue intake, PR hygiene, and release-note quality via templates, path-based labeling, and label-grouped autogenerated release notes.

Changes:

• Added GitHub Issue Forms (bug + feature request) and disabled blank issues.
• Added a PR template checklist to encourage consistent testing/docs/release-note labeling.
• Added a path-based PR labeler workflow plus a release-notes categorization config.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
.github/workflows/labeler.yml	New workflow to apply path-based labels to PRs.
.github/release.yml	Configures categorized auto-generated GitHub release notes by label.
.github/PULL_REQUEST_TEMPLATE.md	Adds PR author checklist (tests/docs/release-note label).
.github/labeler.yml	Defines path-to-label mappings (core/docs/ci/backend:*).
.github/ISSUE_TEMPLATE/feature_request.yml	Adds feature request issue form.
.github/ISSUE_TEMPLATE/config.yml	Disables blank issues.
.github/ISSUE_TEMPLATE/bug_report.yml	Adds bug report issue form capturing backend + versions + reproducer.

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