Add CITATION.cff for Standardized Ibex Citations

Nobody has claimed this yet.

Hi Ibex-team,

I’d like to propose adding a [CITATION.cff](https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files) file to the Ibex repository. This would help researchers and users cite Ibex consistently in academic (and professional) work.
I’ve generated a draft using [cffinit](https://citation-file-format.github.io/cff-initializer-javascript/#/finish) as a starting point:
```
# This CITATION.cff file was generated with cffinit.
# Visit https://bit.ly/cffinit to generate yours today!

cff-version: 1.2.0
title: Ibex
message: >-
If you use this software, please cite it using the
metadata from this file.
type: software
authors:
- name: lowRISC CIC
address: 7 Hills Road
city: Cambridge
post-code: CB2 1GE
country: GB
website: 'https://lowrisc.org/'
abstract: >-
Ibex is a production-quality open source 32-bit RISC-V CPU
core written in SystemVerilog. The CPU core is heavily
parametrizable and well suited for embedded control
applications. Ibex is being extensively verified and has
seen multiple tape-outs. Ibex supports the Integer (I) or
Embedded (E), Integer Multiplication and Division (M),
Compressed (C), and B (Bit Manipulation) extensions.
```

**Open Questions**:

1. **Authorship:** Should lowRISC CIC be the sole author, or would it be more appropriate to include the [PULP Platform](https://www.pulp-platform.org/) as a co-author (or primary author)?

2. **Additional Metadata:** Would you like to add a DOI, ORCID, version, or specific contributors (e.g., key maintainers) to the file?

3. **Preferred Citation Format:** Do you have existing guidelines for how Ibex should be cited (e.g., in papers or documentation, which I have missed)?

I’m happy to refine this further or submit a PR. Thanks for considering this!

Best,
Felix Uhle

Review the linked CITATION.cff documentation and the draft in the issue; confirm the preferred authorship and metadata with maintainers before editing. Add and validate CITATION.cff once those questions are settled, ensuring it accurately represents Ibex and its citation guidance.

Written by the indexing model from the issue text.