Add issue template config and CITATION.cff

[Gero1999]

Description

Adds two small project-health files identified during an OpenSSF Best Practices Badge review (see #1405).

1. .github/ISSUE_TEMPLATE/config.yml

Configures the "New issue" chooser to route users to the right place:

• Disables blank issues so people use the existing Bug / Enhancement templates.
• Adds contact links pointing questions to the Q&A Discussions category and ideas to the Ideas category — matching the guidance already in CONTRIBUTING.md. This reduces misfiled "how do I…?" issues.

2. CITATION.cff

Adds a machine-readable citation file so GitHub shows a "Cite this repository" button with BibTeX/APA export. It mirrors the existing inst/CITATION (aNCA authors + ORCIDs, Apache-2.0, repo/site URLs).

The PKNCA paper (Denney et al., 2015) is included under references: as related work (PKNCA provides the core estimators), not as the primary citation for aNCA.

CITATION.cff is added to .Rbuildignore so R CMD check does not flag it as a non-standard top-level file.

Type of change

• Project-health / documentation files (no package code changed)

Checklist

• No R code changed (no version bump / NEWS entry needed)
• CITATION.cff added to .Rbuildignore
• Citation content mirrors inst/CITATION; PKNCA reference kept separate as related work

Relates to #1405