Add a CITATION.cff file

**Is your feature request related to a problem? Please describe.**
I wanted to cite Indico but I couldn't find any "Cite Us", either in the webpage, either in the GitHub repo

**Describe the solution you'd like**
Add a CITATION.cff at the root of the repo using https://citation-file-format.github.io/cff-initializer-javascript/#/

**Describe alternatives you've considered**
Manually or not citing Indico... :(

**Additional context**
N/A

Create CITATION.cff at the repository root using the linked CFF initializer. Review the generated metadata for Indico and validate that the file is recognized as a citation file; the work is done when the repository provides a usable citation entry.

Written by the indexing model from the issue text.