Add citation metadata #3062

Job	Run time
Tests for downstream project Firedrake	15m 53s
Conda Examples	2m 14s
Documentation	2m 1s
Tests for downstream project arraycontext	2m 46s
Tests for downstream project meshmode	10m 54s
Flake8	23s
Tests for downstream project grudge	7m 25s
Pylint	2m 10s
Mypy	1m 54s
Tests for downstream project pytential	33m 20s
Tests for downstream project pytato	5m 25s
Conda Pytest (ubuntu-latest)	4m 7s
Conda Pytest with Intel CL	4m 7s
Conda Pytest without arg check	4m 14s
Conda Pytest (macos-latest)	12m 23s
Conda Pytest Twice (for cache behavior)	5m 17s
Validate CITATION.cff	7s
	1h 54m 40s

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