Add preferred reference and citation to `sites.json`?

### Description

It occurs to me that there is some overlap between `sites.json` and https://github.com/astrofrog/acknowledgment-generator. They have different use cases, but they are both about keeping a database of observatory data. In `sites.json` we collect the source of the LAT/LON information, which often (but not always) is the same scholary paper that would be used to acknowledge data from that observatory. Thus, it might be good for the community to combine the two data bases in one.

### Additional context

Since it would be considerable work to backfill all values for all observatories that are not common to both databases right now, I suggest to allow a string value of "not filled in the database yet, but please support this work by open a PR here: LINK". There is some overlap already, in particular for the major observatories.

`sites.json` also does not include space-based facilities. Either we change the structure to allow a value for location that is "space", or we keep this restricted to ground-based facilities and leave is up to users such as https://github.com/astrofrog/acknowledgment-generator to handle space facilities separately.

Start by inspecting sites.json and comparing its observatory records with the acknowledgment-generator database. Clarify whether the project should support citation fields, space-based facilities, or a shared schema, then document the agreed scope and completion criteria before implementation.

Written by the indexing model from the issue text.