Consistency with 'references' in docstrings

[Astro-Kirsty]

Currently we have a number of different ways to implement a reference. We should make sure we utilise the same method each time.

Example that links to the actual references page – personally I like this option
https://docs.gammapy.org/1.2/api/gammapy.estimators.TSMapEstimator.html

Full reference in docstring
https://docs.gammapy.org/1.2/api/gammapy.catalog.SourceCatalog1LHAASO.html
https://docs.gammapy.org/1.2/api/gammapy.catalog.SourceCatalog2HWC.html

List of multiple references
https://docs.gammapy.org/1.2/api/gammapy.stats.cash.html
https://docs.gammapy.org/1.2/api/gammapy.astro.darkmatter.ZhaoProfile.html

I suggest we decide on a specific format and then add that to the documentation guide.
https://docs.gammapy.org/1.2/development/doc_howto.html

There are also a number of things that we could add to the documentation guide to ensure we are producing the correct docstrings and examples. Also as a note to everyone, if we are creating new functions or example codes we should make sure to build the docs and the run the test for the docstring examples i.e.
pytest --doctest-modules gammapy/estimators/parameter.py