feat: add `CITATION.cff` file and BibTeX citation

Hello,

I often reference the reproducible-builds.org website in various documents. Historically, I've reused the same citation, copying and pasting it wherever needed. This MR introduces an official citation for the project, aimed at saving time for anyone who needs to reference r-b work without the hassle of assembling the necessary details themselves.

The citation provided here is intentionally brief. The goal of this MR is to lay a foundation; we can refine the citation details incrementally.

This MR provides two things:

1. Adds a CITATION.cff file to the root of the repository:
   A CITATION.cff file facilitates citation of the software or project by providing a standardized format for users and contributors. For more on the benefits, see Why include a CITATION.cff file?.
2. Updates the _docs/publications.md file to include a BibTeX export of the CITATION.cff file. BibTeX is among the most commonly used citation formats in academic and research communities, enhancing the accessibility and ease of citing our project.

Let me know what you think.

Other related work:

• Add cffconvert in Nix: https://github.com/NixOS/nixpkgs/pull/295255

To-do:

• Once it will be in Nix, add cffconvert in the project's flake.nix file